Euxyl K 903

Overview

Euxyl K 903 sits in the same family as Cosgard and Geogard ECT — benzyl-alcohol-based, acid-co-active, no parabens or formaldehyde. It is a pale yellow liquid that combines benzyl alcohol (around 78-84%), benzoic acid (11-13%), dehydroacetic acid (6.5-7.5%), and a small dose of tocopherol (vitamin E) as an antioxidant.

The reason a hobbyist reaches for Euxyl K 903 specifically — instead of Cosgard or Geogard ECT — is its suitability for sensitive areas. It is appropriate for eye-area products and baby care because the actives are all very mild on skin and there is no salicylic acid (which is what disqualifies Geogard ECT from baby products). The vitamin E is a nice bonus: it doubles as a mild antioxidant and helps prevent the slight yellowing that benzyl-alcohol-based blends can develop over time.

Note: the INCI is sometimes confused with other “Euxyl” products. Euxyl K 903 is benzyl-alcohol based. Euxyl PE 9010 is phenoxyethanol-based. They are not interchangeable — they have different pH ranges, different scents, and different best-use cases.

What it does in a formula

Primary role: broad-spectrum preservation in low-pH formulas. Benzyl alcohol handles bacteria. Benzoic acid and dehydroacetic acid handle yeasts and moulds. The blend is effective against both Gram-positive and Gram-negative bacteria as well as common moulds and yeasts.

Secondary role: the tocopherol portion is a mild antioxidant that also helps stabilise oil phases against rancidity over time. Useful in formulas with sensitive vegetable oils.

How to use

Use at 0.4-1.2% of the total formula. Reliable performance sits at 0.8-1%.

Add to the cool-down phase, ideally below 40°C. Euxyl K 903 tolerates short heating up to 80°C if absolutely necessary, but heat above that degrades the benzoic acid quickly. The cool-down rule applies as with all preservatives.

Optimal pH is 5-5.5, and the preservative becomes ineffective above pH 6. Like Cosgard, the acid co-actives only work in their undissociated form, so the acidic window is non-negotiable.

Worth knowing: Euxyl K 903 tends to drop your formula’s pH by 0.75-1.5 points when added — the acids are doing exactly what acids do. Always re-test pH after adding and adjust upward if needed.

Best for / Worst for

Best for: baby creams and lotions, eye-area products, gentle facial moisturisers, leave-on serums at pH 4-5.5. Anywhere you want a preservative that is not phenoxyethanol and not salicylic-acid-containing — these two restrictions show up a lot in baby and sensitive-skin formulations.

Worst for: high-pH formulas (above 6 — for those use Optiphen). Also a poor choice for fully clear watery formulas without solubilisation, because benzyl alcohol has limited water solubility. Avoid in anionic-heavy systems without a co-antioxidant.

Common pitfalls

Confusion with Euxyl PE 9010: as noted, these are different products with different INCIs. PE 9010 is phenoxyethanol-based and works up to pH 12; K 903 is benzyl-alcohol-based and caps at pH 6. Buying the wrong one is a common mistake.

pH drop: Euxyl K 903 will pull your finished pH down. Re-test after adding. If you started at 5.5 and end up at 4.0, that may be too acidic for some skin (citrus territory) and you will need to nudge it back up with sodium hydroxide or sodium bicarbonate.

Using above pH 6: the dehydroacetic acid stops working above pH 6, and the benzoic acid loses most of its activity above pH 5. Above pH 6 you have an expensive scent additive that is not preserving anything.

Solubility in watery formulas: same caveat as Cosgard and Geogard ECT. Use in emulsions or solubilise.

Confusing INCI listings: older product listings sometimes show slightly different ratios. The current, verified INCI is the one in the frontmatter above. If you buy from a small reseller, double-check the actual product spec sheet.

Substitutes

• Geogard ECT — same family, also Ecocert/Cosmos-approved, works up to pH 8 instead of 6. The slightly broader-spectrum alternative for non-baby formulas.
• Cosgard (Geogard 221) — similar profile, slightly less aggressive scent, also fits in pH 6 max.
• Euxyl PE 9010 — entirely different family (phenoxyethanol + ethylhexylglycerin), works at higher pH. Use this if your formula is above pH 6.
• Liquid Germall Plus — much lower use rate, broader pH range, but releases trace formaldehyde — not suitable for baby/eye products if customers are formaldehyde-conscious.