The Korean Vitamin C Serum I Keep Repurchasing
Last October I stared at a bathroom mirror under those brutal fluorescent lights and just thought: when did my skin get so dull?
Not bad, not broken-out, just... grey. Flat. The kind of dull that no amount of highlighter fixes. I'd been using the same Western vitamin C serum for almost a year — the one with the little dropper, comes in a brown glass bottle, half the K-beauty Reddit community swears by it. It's fine. But it's also $42 for 1 fl oz, and honestly I wasn't seeing results anymore.
So I did what I always do when skincare gets boring: I went to the Olive Young on Garosugil and spent about forty minutes reading labels.
A Quick Word on Vitamin C Serums (and Why Korean Ones Are Different)
Here's the thing about vitamin C in skincare — the form of vitamin C matters enormously. Most serums use L-ascorbic acid, which is effective but notoriously unstable. It oxidizes. It turns orange. It can irritate sensitive skin if the concentration is too high.
Korean brands have actually done a lot of interesting work here. Some use stabilized ascorbic acid derivatives (like ascorbyl glucoside or 3-O-ethyl ascorbic acid) that are gentler and longer-lasting. Others lean into high-concentration L-ascorbic acid but pair it with ferulic acid and vitamin E to slow oxidation — which is basically what the fancy Western serums do, just at a fraction of the price.
I tested three. Here's how it went.
The Three I Tested
1. COSRX The Vitamin C 23 Serum — The Heavy Hitter
Yesstyle → | Stylekorean → | Amazon →
I picked this one up first because the name is basically just "23% vitamin C" and I respect that kind of honesty. It's about ₩35,000 / ~$26 for 20ml, which sounds small but you're using maybe 2-3 drops per application.
The texture is what surprised me most. It's thick — almost paste-like, honestly a bit weird to work with at first. You press it in with your fingertips rather than spreading it, and it needs a solid 60-90 seconds to absorb before you layer anything on top. If you rush it and go in with moisturizer too fast, you'll end up with pilling.
I've been using this for about five weeks now and the results are real. My hyperpigmentation from a breakout last autumn has visibly faded — not gone, but noticeably lighter. My overall skin tone is more even, especially around my nose and forehead.
But here's my caveat: this serum oxidizes faster than I'd like. About six weeks in, the color shifted from pale yellow to a deeper amber. It was still fine to use but the irritation risk goes up as L-ascorbic acid oxidizes. If you can't get through a bottle in 6-8 weeks, this might not be the one.
Compared to the SkinCeuticals C E Ferulic (which runs around $166 for the same 20ml) — the COSRX gets you genuinely similar results at about 1/6 the price. I said what I said.
2. TIAM Vitamin C24 Surprise Serum — The Gentler Alternative
Yesstyle → | Stylekorean → | Amazon →
TIAM doesn't get nearly enough attention and I genuinely don't understand why.
This one is priced around ₩28,000 / ~$21 and it uses a stabilized form of vitamin C (3-O-ethyl ascorbic acid) alongside niacinamide. The texture is a proper lightweight serum — watery, absorbs in about 20 seconds, no stickiness. Smells faintly citrusy in a way that actually smells like real citrus, not the weird vitamin-y smell some high-concentration serums have.
I used this for three weeks after finishing a bottle of the COSRX and honestly? My skin didn't freak out even once. Zero tingling, zero redness. For anyone who's ever tried a high-percentage L-ascorbic acid serum and had a bad time — this is probably your answer.
The trade-off is that the brightening results are slower and more subtle. After three weeks I could see improvement but it was more of a "glow from within" effect than the targeted dark spot correction the COSRX gave me.
3. Some By Mi Galactomyces Pure Vitamin C Glow Serum — The Wild Card
I picked this up on a whim from the Olive Young near Hongdae station — it was in a little end-of-aisle display and caught my eye. About ₩22,000 / ~$16, which made it the most affordable of the three.
It combines galactomyces ferment filtrate (a brightening ingredient that's huge in Korean skincare right now) with vitamin C. The gel texture is really pleasant — cool on application, absorbs quickly, leaves a subtle plumping effect.
But honestly, I didn't see as dramatic results with this one. It's more of a general brightening serum than a dedicated vitamin C treatment. Good for someone who wants to ease into actives without committing to a high-percentage serum. Not what I needed.
Side-by-Side Breakdown
| COSRX C23 | TIAM C24 | Some By Mi | |
|---|---|---|---|
| Vitamin C type | L-ascorbic acid | 3-O-ethyl ascorbic acid | Mixed (+ galactomyces) |
| Concentration | 23% | 24% | Lower / undisclosed |
| Texture | Thick/paste | Lightweight watery | Gel |
| Absorption time | 60-90 sec | ~20 sec | ~30 sec |
| Results speed | Fast (2-3 weeks) | Gradual | Subtle |
| Skin sensitivity | Not great for sensitive | Great for sensitive | Fine for most |
| Oxidation concern | Yes (6-8 weeks) | Low | Low |
| Price | ₩35,000 / ~$26 | ₩28,000 / ~$21 | ₩22,000 / ~$16 |
How I Use Them (Routine Notes)
Vitamin C goes on after cleansing and toning, before moisturizer. That's non-negotiable — it needs to be closest to clean skin to absorb properly.
I use the COSRX in the morning (vitamin C + SPF in the AM is the brightening combo) and skip it if I'm using retinol at night. On the nights I do vitamin C again, I'll use the TIAM instead since it's gentler.
Do NOT mix vitamin C directly with niacinamide, AHAs, or BHAs in the same application step. Layer them, give each one a minute to absorb, or use them at different times of day. I learned this the hard way — combined them too fast once and ended up with flushed, irritated skin that took two days to calm down.
So Which One?
Honestly, it depends on what you're trying to fix.
If you have stubborn dark spots or hyperpigmentation: Go with the COSRX C23. The results are real and the price-to-performance ratio compared to Western alternatives is genuinely hard to argue with. Just commit to finishing the bottle within 6-8 weeks.
If you have sensitive skin or you're new to vitamin C serums: The TIAM C24 is the move. It's gentler, more forgiving, and the stabilized vitamin C form means you don't have to stress about the bottle going off.
If you want a low-maintenance glow without dealing with actives anxiety: The Some By Mi is fine. Good for mornings when you want something quick and easy.
I'm currently on my second bottle of the COSRX and a backup TIAM for the nights my skin is being dramatic. That's basically the answer.
Where to Buy
All three are easy to find internationally:
- COSRX C23: Yesstyle → | Amazon →
- TIAM C24: Stylekorean → | Yesstyle →
- Some By Mi: Yesstyle → | Stylekorean →
Disclosure: This post contains affiliate links. If you purchase through these links, I may earn a small commission at no extra cost to you. I only recommend products I've personally tried and genuinely like.