A stable vitamin C serum comes down to three decisions: which form of vitamin C you use, the pH of the formula, and the packaging it lives in. L-ascorbic acid is the most studied form but oxidises quickly and works best at a low pH (below about 3.5). Derivatives are gentler and more stable but less potent per percentage point. Most indie brands in NZ and Australia launch well with either, provided stability testing happens before the first batch.
Vitamin C is one of the most requested actives we see at The INCI Lab, and one of the easiest to get wrong. Below is how we think about it when a founder arrives with a brief, whether they are in Auckland, Melbourne or anywhere between.
Which form of vitamin C should my serum use?
There are two broad families. L-ascorbic acid (LAA) is the direct, active form. Published guidance generally puts useful levels at roughly 10 to 15% at a pH below 3.5, with benefits tailing off under about 5% and irritation becoming more likely above about 20%. It is potent, but it degrades in light, heat and oxygen.
Derivatives such as magnesium ascorbyl phosphate, sodium ascorbyl phosphate, ascorbyl glucoside and tetrahexyldecyl ascorbate (THD ascorbate) are far more stable and can be formulated at a more skin-friendly pH. The trade-off is that they need to convert in the skin, so they are usually less potent per percentage point. THD ascorbate is oil-soluble, which suits anhydrous or oil-based serums.
Neither is better in the abstract. LAA suits a brand built on performance and a clinical feel. Derivatives suit sensitive-skin positioning, longer shelf life and lower return risk.
Why does vitamin C serum go brown?
Oxidation. LAA breaks down into dehydroascorbic acid and then brown by-products when exposed to oxygen, light, heat and higher pH. A serum that darkens on a bathroom shelf is not necessarily unsafe, but it has lost potency and it looks unappealing to customers.
Formulation can slow this down: chelating agents to bind trace metals, antioxidant partners such as vitamin E and ferulic acid, and careful control of water activity and pH. It cannot stop it, which is why packaging and honest shelf-life labelling matter just as much as the recipe.
What packaging works for vitamin C?
Opaque or dark glass limits light exposure, and an airless pump limits the oxygen a dropper reintroduces every time it is used. A clear glass dropper bottle looks beautiful in a flat lay but is one of the fastest ways to shorten shelf life.
Low pH formulas can also interact with pump components and gaskets, so compatibility needs to be checked, not assumed. Our guide to cosmetic packaging compatibility testing explains what to look for.
How do you test a vitamin C serum for stability?
Because vitamin C is reactive, we treat stability testing as non-negotiable. That means checking colour, odour, pH and viscosity over time at different temperatures, ideally in the final pack, and where possible measuring the actual vitamin C content rather than relying on appearance alone. Read more in why 12-week accelerated stability testing matters.
Preservation also needs thought. Low pH helps, but a water-based serum still needs a validated preservative system, so ask us about preservative efficacy testing.
What can I claim on a vitamin C serum in NZ and Australia?
Keep claims cosmetic: brightening the look of skin, supporting a more even-looking tone, antioxidant protection for the skin surface. Claims that imply treating a condition, such as treating pigmentation disorders or repairing sun damage, risk moving the product toward therapeutic territory and need substantiation. Our article on what you can legally claim on a skincare label covers the boundary.
For ingredient compliance, confirm each vitamin C form on your formula is appropriate for your target markets (NZ, Australia under AICIS, EU, US) before you commit to packaging. We include this in the compliance documentation for every project.
How does The INCI Lab develop a vitamin C serum?
It depends on how custom you want to go:
- Ask The Chemist: a 30-minute consult from $150 to choose between LAA and a derivative and sanity-check your brief.
- The Accelerator: a semi-custom fast-track over roughly 4 to 6 months, from $2,000 to $5,000.
- The Visionary: fully bespoke formulation over roughly 9 to 12 months, from $4,000, with full IP transferred to you.
- Ready-to-Go: pre-formulated, stability-tested bases if you want to move quickly.
Small-batch manufacturing starts at minimums of 100 to 500 units, so you can launch without a warehouse of stock. We are cruelty-free, vegan-capable and can formulate fragrance-free, and we prepare compliance documentation for NZ, Australia, the EU and the US.
Frequently asked questions
What percentage of vitamin C should a serum contain?
For L-ascorbic acid, roughly 10 to 15% at a pH below about 3.5 is a common working range. Derivatives are used at different levels depending on the form, and your chemist will set this against your skin-feel, stability and positioning goals.
Is L-ascorbic acid or a derivative better for a first product?
Derivatives are usually the more forgiving first launch because they are more stable and gentler. L-ascorbic acid suits brands that want maximum potency and are ready to invest in airless, opaque packaging.
How long does a vitamin C serum last?
It varies by form, pack and storage. Formulas built on L-ascorbic acid typically have a shorter usable life once opened than derivative-based serums. Stability data from your own pack is the only reliable answer.
Do I need stability testing for a vitamin C serum?
Yes. Vitamin C is one of the most reactive actives you can use, and colour, pH and potency changes are common. Testing before launch protects your customers and your reputation.
Can I make a vegan, cruelty-free or fragrance-free vitamin C serum?
Yes. All three are straightforward with vitamin C, and we can formulate and document them for NZ and Australian brands.
What is the minimum order for a custom vitamin C serum?
Our small-batch minimums sit between 100 and 500 units depending on pathway and pack. Get in touch to talk through your numbers.
Pricing shown is a starting guide only. Ready to brief your serum? Book a 30-minute chat with our chemist.