Vitamin C in skin care: ascorbic acid, derivatives and stability
Ascorbic Acid, Sodium Ascorbyl Phosphate, Ascorbyl Glucoside, Tetrahexyldecyl Ascorbate: the many INCI names of vitamin C, why the pure form is so unstable and what a colour change in your serum means.
Updated: 31 October 2026 · Reviewed by: Kamil Pyziak
Why vitamin C is in so many serums
Vitamin C is an antioxidant: it can neutralise reactive molecules that form, among other things, under UV exposure and air pollution. In skin care it is one of the most widely researched ingredients and is commonly used in products for dull or uneven-looking skin and in “antioxidant” day serums.
“Vitamin C” on the front of a pack, however, can mean very different raw materials. Some are pure vitamin C; others are derivatives designed to be more stable. To know which one you have, you need the INCI list.
Ascorbic acid: the original, and the most fragile
The pure form is listed as Ascorbic Acid (L-ascorbic acid). It is the form the body itself uses and the one with the longest research history in skin care. Its weakness is stability: in water, and especially in contact with air, light, heat and traces of metals, it oxidises fairly quickly and loses its activity.
Formulas with ascorbic acid as the main active are usually clearly acidic, because a low pH helps keep it stable. That is one reason why such serums can tingle on application. An alternative is to avoid water altogether: anhydrous serums, silicone- or glycol-based gels and powders that you mix in just before use are all ways around the problem.
Derivatives and their INCI names
Derivatives are modified forms of vitamin C that hold up better in a formula. Water-soluble ones include Sodium Ascorbyl Phosphate and Magnesium Ascorbyl Phosphate (phosphate salts that work at a milder pH), Ascorbyl Glucoside (vitamin C bound to glucose) and 3-O-Ethyl Ascorbic Acid. Oil-soluble ones include Ascorbyl Palmitate and Tetrahexyldecyl Ascorbate, which you may also see under the older name Ascorbyl Tetraisopalmitate.
Derivatives are generally expected to release or behave like vitamin C after reaching the skin. How efficiently this happens, and how much research supports each one, differs from derivative to derivative. “More stable” therefore does not automatically mean “stronger” – it means a different trade-off between stability, comfort and evidence.
How formulators keep vitamin C stable
Look at the rest of the list for clues. Antioxidant partners such as Tocopherol (vitamin E) and Ferulic Acid are often combined with ascorbic acid because they support each other. Chelating agents such as Disodium EDTA or Sodium Phytate bind metal traces that speed up oxidation. A low water content or an anhydrous base also helps.
Packaging is part of the formula: opaque or dark glass, airless pumps, small volumes and single-dose capsules all limit contact with light and air. A serum that turns from clear or pale yellow to orange or brown has generally oxidised; some formulas have a natural tint from the start, so compare with how the product looked when you opened it.
Vitamin C as a helper ingredient
Not every vitamin C on a list is there as the “star”. Ascorbic Acid and Sodium Ascorbate are also used in small amounts as antioxidants or pH adjusters, and Ascorbyl Palmitate is a classic antioxidant that protects oils from going rancid. When these names appear near the end of a long list, they are usually protecting the formula rather than targeting the skin.
Vitamin C is also not a sunscreen. It does not replace a product with UV filters; antioxidant serums are typically used underneath sun protection, not instead of it.
Practical notes
Acidic vitamin C serums can sting or cause redness, particularly on sensitive skin or when used alongside other strong actives such as exfoliating acids or retinoids. Brands often suggest introducing such products gradually and storing them closed, away from heat and sunlight, and using them up within the period shown by the open-jar symbol.
If your skin reacts persistently, or if you have a skin condition or are using prescription treatments, ask a pharmacist or doctor before adding a vitamin C product to your routine.
What the label does not tell you
The INCI list shows which form of vitamin C is used, but not its concentration, the pH of the formula or how much of it is still intact after months on a shelf. A percentage on the pack is the manufacturer’s statement, and for derivatives it is not directly comparable with a percentage of pure ascorbic acid.
Freshness matters more with vitamin C than with most ingredients. The production date, the period after opening and the way you store the product all affect what is actually left in the bottle.