Review: Tiam My Signature C Source Vitamin C Serum (formerly O.S.T C20)

So first a little back story…Tiam are a company that I only became aware of a few months back, when I saw that they would be at the K Beauty Expo in Seoul in May, and I learned that they are little sister company of Cosrx. Needless to say I was intrigued, having been a huge fan of Cosrx for the past few years. 

*This product was kindly gifted to me, but this does not affect my review in any way. See my disclaimer tab for more details of how I collaborate.

I met Luna from Tiam when I visited their booth at the expo, and she very kindly gifted me with some products of my choice (the two Bubble Packs and the Anti-Pollution Salt Facial Scrub). After testing the products, I was suitably impressed.

And then, about a month ago, something caught my eager eye- Tiam had rebranded the famous O.S.T. Original Pure Vitamin C20 serum and were selling it. Having just dipped my toes in the Vit C pool, and been overwhelmingly excited at the brightening effects, I was ready to plunge in head first and buy this 20% serum. But duh, Katherine, of course it’s not available in Korea because of the strict laws (EDIT: this is now being sold in Korea, woohoo!). That’s when Tiam offered to send me the product in return for honest reviews. So here I am, almost four weeks in, and here you are, ready to read my review…lesgo.

Vitamin C serums are predominantly used for brightening, helping reduce PIH and acne scars, light exfoliation, and overall evening of skin tone. Whilst my skin is already reasonably bright (as a result of my three year affair with my k-beauty routine), I frequently deal with mild PIH and red marks from small pimples. My skin tone is also quite uneven with redness, and it’s something I’m conscious of.

The packaging states:

Tiam My Signature C Source delivers a multitude of complexion benefits for beautifully healthy skin. Pure Vitamin C refine, brighten, balance, and tone skin for a radiant appearance.

Product Details and Ingredients

Full name: My Signature C Source

Amount: 30ml

Ingredients: (No changes from the original C20 product)

Water, Ascorbic Acid, Alcohol, Sodium Lactate, Butylene Glycol, Glucose, PEG-60 Hydrogenated Castor Oil, 1,2-Hexanediol, Phenoxyethanol, Sodium Hyaluronate, Bis-PEG-18 Methyl Ether Dimethyl Silane, Xanthan Gum, Diethoxyethyl Succinate, Ammonium Acryloyldimethyltaurate/VP Copolymer, Carthamus Tinctorius (Safflower) Flower Extract, Citrus Aurantium Dulcis (Orange) Peel Oil, Glycerin, Zinc PCA, Panthenol, Niacinamide, t-Butyl Alcohol, Camellia Sinensis Leaf Extract, Beta-Glucan, Tocopheryl Acetate, Lecithin, Caprylic/Capric Triglyceride, Ubiquinone, Diisopropyl Adipate

So with 20% Ascorbic Acid, it contains about the highest concentration of Vitamin C you can buy, which is great! Next is alcohol. Now personally, I don’t have a problem with alcohol in certain formulas, and I’m okay with it in this, but of course it’s a personal choice. There’s also a few other ingredients I’m happy to see- sodium hyaluronate and camellia sinensis leaf extract in particular, for moisturizing and antioxidant properties.

Niacinamide is featured here, which is really quite confusing, considering its widely thought that using niacinamide and Vitamin C together could be problematic. There has been some discussion on this, and I don’t pretend to know the science behind it. Luckily we have highly knowledgeable bloggers like Stephen at to help us non-science-minded folks understand the science behind what we’re putting our faces. His article about Vitamin C and niacinamide really helped me, and I’ve decided to allow a short wait time between using the serum, and any niacinamide product (virtually all of my products contain niacinamide). I’m not too concerned about using them together, nor the small (?) amount of niacinamide in this product.

How to use:
You have to be quite particular with how you use and store Vitamin C, and I’ll admit I felt a bit nervous initially.
Vitamin C can oxidize very quickly, and should therefore be stored in a cool, dark place. I keep mine in the refrigerator, in its box. It does come in a dark glass bottle to help protect the product.

For use, I drop several drops directly onto a clean face, about 15 minutes after I’ve cleansed. I expected a slight tingling sensation, but this just seems to happen occasionally, and oddly only on my forehead- I guess my skin is more sensitive there. The tingling only lasts a couple of seconds for me. For sensitive skin, it is suggested to mix the serum with a toner. I had been using 10% Vitamin C pads beforehand, so this may have helped build my tolerance, and I’m sticking with using the serum alone.
It definitely needs mentioned that there’s no room for slacking on sunscreen application if you’re using this serum. As I already use minimum SPF factor 50 PA+++ sunscreens every damn day (and the correct recommended amount-so important), this wasn’t a concern for me, but something you need to be aware of if you don’t religiously use sun protection. I do use this in the morning despite the night time recommendation, because of my acid use at night. But I’m rarely in the sun for long on a daily basis, only for about 15 minutes during my commute.

Fact, I love dropper serums. They’re hygienic, easy to use, and easy to control amount of product. As I mentioned earlier, the dark colored glass also helps protect the product from oxidization. 

The amount of serum provided (30ml) is reasonable, seeing as the product should be used up fairly quickly.

Texture and Scent
This scent, man, I love it. It’s a fresh and fruity, citrus orange scent, and it really hits you when you first use the product. It fades quite quickly, but that first whiff is glorious.
The texture is a thick, clear, classic serum texture and spreads smoothly and easily. However, as noted in many of the reviews I’ve seen, it does leave a distinctly sticky residue. It’s not pleasant. BUT, oddly, as soon as you layer products over the top, the stickiness completely disappears and it doesn’t affect the rest of my routine or make up application. It’s really bizarre, and there’s no heaviness at all. Used on it’s own- not the best. Used within a routine (as I always do)- no problem for me.

Gotta admit, it lived up to all of my expectations. As I said, my skin is already fairly bright, but I’ve noticed SERIOUS overall glow coming through over this four week testing period that wasn’t there before I started using the serum. The before and after photos actually really surprised me because I hadn’t realized how dramatic the brightening has been. FYI I’ve been using it almost every morning.

BEFORE: Taken July 13th at 9pm, with a cleansed and toned face.

AFTER: Taken August 10th at 11pm, with a cleansed and toned face.

You can definitely see some of the redness and also some of my freckles are significantly lighter and my skin is generally more even-toned.
It has also helped fade pimple marks significantly quicker. When I break out on my forehead or on my chin, the red marks tend to linger and irritate me (you can see the red PIH on my chin in the photos above, and how it’s more even and faded in the later photos). With the serum, the marks fade so much faster. The same can be said for the pimples I experience on my chest. I break out on my chest more than anywhere else, and I’m often left with red marks where the spots have been. I’ve been testing the serum here as well as my face, and the results have been even more dramatic with fading the marks so quickly. 

In terms of exfoliating, my skin does feel slightly smoother. I don’t think Vitamin C has particularly affected my (more frequent, as of late) breakouts in terms of prevention, but rather just helped control the redness and marks from those breakouts, which is what I expected. I didn’t use my BHA or AHA for two weeks when I was testing this, and I definitely needed to reincorporate those to help with my pores and skin texture, and use them at night, whilst using the Vitamin C in the morning. Using all three together has been perfect. 

I have zero complaints about the serum…it’s reasonably priced, well packaged, and provided me with some fabulous brightening results. It’s helping keep the redness in my skin at bay, and fading any marks so much quicker. It’s easy to use, and, with my schedule, can be slotted into my regular routine easily. I’m super happy with it, and will def never be without Vitamin C in my routine again!

Have you used the Tiam/OST serum before? How did it work out for you?

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  1. Hi Katherine! You said you use yours every morning because there are acids in your nighttime routine? Did I get that right? What acids do you use at night?

    1. Hi! Yep, although actually I’ve juuuust decided to change to acids in the morning and the vitamin c at night. Im currently using gentle acids- Cosrx BHA Blackhead Power Liquid, and the Wish Formula Day Day 365 pads 🙂

    1. Nice! That’s great that it worked so well for you. I’m the same, I’ve kept it in my routine and don’t think I’ll ever be without it!

    1. Hi! Yes I do, every morning I use it on my nose, chin and forehead. It’s only 4% BHA so my skin can tolerate every day with no problem, but it will depend on your skin!

  2. Hi, I just bought the same but what are the products you matched and applied to you face. I’ve done my researched and it says not to use any AHA, BHA, Retinal, Betaine akong with the Vitamin C but can’t figure out what are the Korean products that perfectly match with the Vitamin C that doesn’t cause any harm to my skin. Appreciate your help

    1. Hi! So yes it says not to use other acidic products because it may irritate your skin. Personally, I use gentle BHA and AHA products at the opposite time of day to the Vitamin C, and it’s fine for my skin. But I can’t recommend because everyone’s skin reacts so differently to actives. I’d suggest starting with very gentle, low % acids at first and then you can work up. Hope that helps!

  3. Hello! I read your review and the results were amazing, been dying to try this product. Is this okay to be used with Cosrx Salicylic Acid Daily Cleanser? I plan to use it only at night, but will I need to still use sunblock in the morning?

    1. Hi! Yes it should be fine to use with that cleanser, the cleanser actually probably has very little effect from the acid because you wash it straight off. Acids need time to work. So (in my opinion, I’m not professional!), it should be fine.
      And yes you will definitely need to use sunblock, you need to use it everyday if this product is in your routine, day or night. Hope that helps!

    2. Hi. Also use Salicylic Acid Daily Cleanser by Cosrx AM & PM. I use Tiam Vit C only at night and it does a great job. I don’t find any problems on my skin with this combination.

    1. Hi! I don’t use the cream anymore but I used to. With the cream I just used it as any other moisturizer. The Liquid I use on a bare face, about 15 mins after cleansing, and then leave for 5-10 minutes before carrying on with my routine. Hope that helps!

    1. Hi! I think so but I can’t find information that says it definitely is, I’m sorry. I’d suggest contacting the company for more information to confirm this.

  4. Hi i got a lot of i fo from here but it
    confused me too.. some says that AHA BHA can’t be used with Vit C…. but as you had tried… can i use this serum with my toner now which is cosrx aha bha toner?

    1. Hi! I think it’s suggested not to use them together because it may irritate skin. I know a lot of people use them together, and yes I do. It really depends on your skin I think. You could start gently testing and see how your skin reacts? I’m not professional so I don’t want to advise you either way, I think best just to see how it works for you! X

  5. Hello! I just want to share my experience about this serum. I’ve been using this already for a couple of weeks. Yes, it really does brighten my skin and also, my blemishes were starting to fade off. But, one thing I don’t like is that I felt mild irritation and itchiness around my tiny bumps on the forehead, or anywhere on my face. Though it is tolerable, i’m just concerned if i should still continue with this one. It seems that my pimpies on my forehead are multiplyng. It really bothers me a lot. 😣

    1. Hi! Thanks for sharing your experience! I know a lot of people do experience tingling/burning from such high % of Vitamin C, and for some people its just too high to use. When I break out I don’t apply the serum on open pimples because it stings so bad. I guess you just need to go with what you think for your skin, whether its worth continuing or its just not working out for you?

      1. I think the best thing to do is not to apply the serum on my tiny bumps. Anyway, i’m so happy and delighted for giving your opinion regarding of my concern. Thank you and more power! God bless ❤

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