Colloidal gold nanocrystals were found to be efficient photocatalysts for the oxidation of thiols to the corresponding disulfide in aqueous solution under UV radiation, although no noticeable catalytic effect of gold nanocrystals was observed under dark conditions for the same process. The turnover number per particle increased with the size of the nanocrystals and reached 1200 molecules/s for 45-nm gold nanocrystals. The photochemical instability of colloidal gold nanocrystals coated by thiol ligands was found to be determined by the photocatalytic oxidation of the thiol ligands.