The present application relates to a cosmetic composition intended for treating keratin fibres, comprising, in a support which is suitable for keratin fibres: (a) at least one enzyme of 2-electron oxidoreductase type in the presence of at least one donor for the said enzyme; (b) at least one anionic amphiphilic polymer containing at least one hydrophilic unit and at least one allyl ether unit containing a fatty chain; as well as to processes for treating keratin fibres, in particular processes for dyeing, permanently reshaping or bleaching the hair, using this composition.