The invention concerns a composition for topical application for extemporaneous preparation, comprising ascorbic acid and a suitable support, the ascorbic acid being obtained by bringing at least one ascorbic acid precursor with the exception of its esters into contact with at least one enzyme that is capable of converting said precursor. It also concerns the use of one or more enzymes selected from L-galactono-1,4-lactone dehydrogenase, L-galactose dehydrogenase, L-sorbosone dehydrogenase, L-gulono-1,4-lactone oxidase and mixtures thereof, or of an extract comprising said enzyme, for the preparation of ascorbic acid for topical use, and a cosmetic method for treating the skin consisting of applying to the skin, either simultaneously or successively, at least one enzyme that can convert an ascorbic acid precursor into ascorbic acid, and at least one ascorbic acid precursor with the exception of its esters.