Tetrahexyldecyl Ascorbate
Also known as Tetrahexyldecyl Ascorbate, THD Ascorbate, THDC, THDA, VC-IP, Vitamin C tetrahexyldecyl ester
“CIR Expert Panel says: safe as used in cosmetics.”
Tetrahexyldecyl Ascorbate (THA; CAS 1445760-15-5) is a lipid-soluble ester of L-ascorbic acid (vitamin C) that functions as a skin-conditioning antioxidant and collagen-synthesis cofactor in cosmetics. The CIR Expert Panel assessed THA as one of 7 ethers and esters of ascorbic acid and concluded the group safe in present practices of use and concentration (Johnson et al., IJT 41(S2):57S-75S, 2022; PMID 35549580), confirmed in the December 2025 QRT (Finding=S). THA's lipophilicity confers superior dermal penetration and formulation stability compared to free ascorbic acid — it remains stable at neutral pH and in anhydrous or emulsion bases, making it the preferred vitamin C form for oil-rich and water-free formulations. Use data submitted to CIR indicated THA is used in 397 formulations at concentrations up to 3% leave-on and 2.5% rinse-off, with a 10% clinical study showing no adverse reactions and evidence of new collagen formation.
Lipid-soluble vitamin C ester: THA's four hexyldecanoate chains make it oil-compatible and capable of penetrating the stratum corneum's lipid bilayers far more efficiently than hydrophilic ascorbic acid, enabling effective dermal delivery without requiring low-pH formulation
Superior formulation stability: unlike ascorbic acid (which degrades rapidly above pH 3.5 and in the presence of oxygen), THA is stable across a wide pH range in both anhydrous and emulsion bases, enabling use in serums, oils, and cream formulations without color-change or oxidation instability
Antioxidant and collagen synthesis cofactor: once hydrolyzed to ascorbic acid by skin esterases, THA delivers the same ROS-scavenging, tocopherol-regenerating, and prolyl/lysyl hydroxylase cofactor activity as free ascorbic acid; a 10% clinical application showed no adverse reactions and histologic evidence of new collagen formation
CIR Expert Panel concluded THA safe in present practices of use and concentration:57S-75S, 2022; December 2025 QRT Finding=S) — group assessment covering 397 commercial formulations, leave-on up to 3%, rinse-off up to 2.5%
CIR assessed THA as part of a 7-ingredient group (ethers and esters of ascorbic acid); the 2022 report is the first CIR safety assessment for THA and postdates the 2017/2018 QRT — THA first appeared in the December 2025 QRT update
At very high concentrations in the presence of free transition metal ions, ascorbate esters may participate in Fenton-type prooxidant chemistry analogous to ascorbic acid; not a concern at typical cosmetic use levels below 3%
Ascorbyl esters hydrolyze to free ascorbic acid in vivo via cutaneous esterases; THA's bulky four-chain ester group (2-hexyldecanoyl) may limit hydrolysis rate compared to smaller ascorbate esters, potentially reducing effective vitamin C delivery relative to the applied dose
CIR Quick Reference Table (December 2025) — Tetrahexyldecyl Ascorbate row: Finding=S, Citation=IJT 41(S2):57S-75S, 2022
“Tetrahexyldecyl Ascorbate | S | IJT 41(S2):57S-75S, 2022”— QRT-Update-Dec2025.pdf, p. 454