How to Choose a Vitamin C Serum: What the Label Should Tell You

Before buying a vitamin C serum, check three things on the label: which form of vitamin C it uses, what concentration it is used at, and — for L-ascorbic acid — the pH. If a brand publishes none of the three, you cannot compare the product to any other, and price tells you nothing.

That sounds obvious. In practice most serums on the US market publish none of it, and the ones that do are easy to shortlist in about a minute.

The form matters more than the marketing

“Vitamin C” on the front of a box can mean several different molecules, and they do not behave the same way:

• L-ascorbic acid — the pure form, the most researched, and the least stable. Needs a low pH to work and oxidises quickly once opened.
• Sodium ascorbyl phosphate (SAP) — a stable derivative, gentler, effective at lower concentrations.
• Magnesium ascorbyl phosphate (MAP) — another stable derivative, often chosen for drier skin.
• Ascorbyl glucoside, THD ascorbate — stable derivatives that convert to ascorbic acid on the skin.

None of these is simply better than the others. But a brand that will not tell you which one is in the bottle is asking you to buy on trust alone.

Concentration, and the 1% problem

US cosmetic labelling requires ingredients above 1% to be listed in descending order by weight. Below 1%, order means nothing. So if vitamin C appears after the preservative on the INCI list, it is present — but not necessarily in a meaningful amount.

Our guide to reading an ingredients list explains how to spot that 1% line. The short version: the first preservative or thickener you recognize marks it.

Typical working ranges quoted for the common forms are 10–20% for L-ascorbic acid and 5–10% for the stable derivatives. A brand publishing a number is not proving efficacy — but it is giving you something to compare.

pH, and why it only matters sometimes

L-ascorbic acid needs an acidic formula to remain in the form that works — commonly cited as below pH 3.5. That low pH is also why L-ascorbic acid serums sting some people. The stable derivatives do not have the same requirement, which is much of their appeal.

If a brand sells an L-ascorbic acid serum and will not state a pH, that is a meaningful omission rather than a technicality.

What good packaging tells you

Vitamin C oxidises on contact with light and air. A clear dropper bottle is a worse container than an opaque or airless one, and a serum that has turned deep orange or brown has oxidised — it is not dangerous, but it is past its useful life.

This is one of the few cases where packaging is a functional claim rather than a design choice.

A one-minute shortlist

1. Can you find the form of vitamin C? If not, stop.
2. Can you find the concentration? If not, you cannot compare it to anything.
3. If it is L-ascorbic acid, is a pH stated?
4. Is the full INCI list published on the product page, not just “key ingredients”?
5. Is it in opaque or airless packaging?

A product that fails the first four is not necessarily bad. It is unassessable, which is a different problem and one you are paying for.

Common questions

Is a higher percentage always better?

No. Above the usual working range, studies generally show no added benefit and irritation rises. More is not the goal; a stated, sensible number is.

Can I use vitamin C with niacinamide?

Yes. The old warning about the two cancelling out came from research at high heat that does not reflect how finished formulas behave on skin.

Morning or night?

Morning is conventional, under sunscreen. Either works — consistency matters more than timing.

Our Vitamin C Serum publishes its form and concentrations on the product page, as does everything else in the range. You can browse the ingredient index to read what each active does before you buy.