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NOW READING This Cult Fave Vitamin C Serum Is Back — Rebranded But With the Same Formula
October 10, 2018

This Cult Fave Vitamin C Serum Is Back — Rebranded But With the Same Formula

When K-beauty brand Tiam announced that they had rebranded the cult vitamin C serum OST C20, but kept the same formulation, skincare lovers everywhere rejoiced. Why you need to get on the Tiam My Signature C Source train.


Listen. I am here to tell you some very important news.


One of the most magical skincare products in the history of the universe is back: the OST C20 serum. Except now it’s rebranded as Tiam My Signature C Source vitamin C serum.


If you have read my blog, The Wanderlust Project, then you will know that many, many, maaaaaaaany moons ago, I wrote about my love for the OST C20. The review is so old, I think I referenced my skin looking like an Instagram filter (hashtag Valencia — and quick side note, who even Instagram filters anymore!?)


And even after all of these years, my feelings about the now-Signature C Source have remained unchanged. It is still everything. FOREVER. It is still bae. An icon. A legend. It’s one of the few K-beauty cult favorites whose formula has remained untouched, even with a rebranding. That’s how great it is.


tiam signature c source


So let’s back up. What is the Signature C Source? Well, it’s a highly potent, extremely effective vitamin C serum. Vitamin C is one of the most important skincare products you can use. It can brighten, get rid of blackheads, improve PIH scarring, help produce collagen for plumper and firmer skin, reduce fine lines, protect the skin against free radicals — it does it all. If you want to have gorgeous skin without all the hassle, then you need to go ahead and pick yourself up a bottle and get your life. Without a doubt, this vitamin C serum is easily one of the best and most effective that I have tried — and I’ve tried a lot.



The Signature C Source contains a whopping 20% vitamin C (ascorbic acid), along with safflower extract, Camellia sinensis leaf extract, and orange peel oil. You can use in the day or night, but I prefer to use it at night because it is a tad on the sticky side. It’s an unfortunate side effect of an otherwise amazing product. Instructions say to use after you apply toner, but I like to use it right after I wash my face and leave it on for about five minutes so it can really do its thing. It can sting a little upon application, but that’s normal and won’t last very long. As soon as the next day, you’ll notice your skin has an otherworldly glow.


I do not recommend using this product with high percentage AHAs, BHAs, retinoids, or any exfoliant. This stuff is pretty powerful on its own. Also, make sure that you store your serum in the fridge or a very cool, dark place. You don’t want to keep this in the bathroom, as it can quickly go bad like most serums with ascorbic acid.



Tiam is a K-beauty brand that uses active ingredients for very targeted skincare — think products specifically targeting brightening, acne, dryness, and dullness. This means that with their products, you can expect real results with very minimal effort. That’s something that I like to hear. And while Tiam is most popular for the Signature C Source serum, they do make other products, like the My Signature A+ Cream (another big mood), bubble packs, peeling toners, facial scrubs, and no-sebum creams. I can’t wait to try more from the brand! If they are anything like my holy grail COSRX, I’m sure that I’m going to love every single thing.


Have you tried the Signature C Source? Or any Tiam products? What did you think? Let me know in the comments!



Sheryll Donerson got her start as a beauty writer by writing K-beauty reviews for her blog, The Wanderlust Project. These days, she's lifting heavy weights, eating tacos, drinking (too much) coffee and is 1/4 of the beauty podcast, Beauty Beyond Basics (or Triple Bees for short). You can find her on Instagram and Twitter at @sheryllrenata.




I've had two tries at vitamin C products, one of them the OST C20, and both times and with both products I managed to overexfoliate and it took forever to get my barrier back. At this point I'm honestly too scared to try again. If I get more courage at some point, I'll definitely try with an exaggeratingly gentle vit C product and err on the side of too little too seldom that time.


I have my eye on this vitamin C product hardcore. I'm trying not to start any new acids until I'm all purged out and adjusted to azelaic acid, but it's haaaaaaard ... Super glad y'all carry this, and I need to remember it'll still be here in a few months!


The new Tiam Face Guard Cream is soooo good. Just light but moisturizing and perfect for transition weather. I can't wait to try the Vita B5 Toner and the powders!