The natural peroxidase and catalase of tea leaf were shown to be active under the conditions employed for the fermentation of minced tea leaf in aqueous suspension. Fermentations carried out under nitrogen with ethyl hydrogen peroxide, which is not decomposed by catalase, yielded predominantly thearubigins, a high proportion of which were polymeric in nature. Fermentations using an excess of hydrogen peroxide gave similar results. Fermentations under conditions of controlled dissolved oxygen using both air and hydrogen peroxide resulted in a pigment distribution intermediate between those observed using air alone and using ethyl hydroperoxide under nitrogen. Adjustment of the amount of hydrogen peroxide used to around 120 μmol g−1 leaf resulted in a pigment distribution closely similar to that of a good quality black tea. A mixture of theaflavins was stable in the presence of air and tea polyphenoloxidase but decomposed rapidly to yield polymeric materials under the action of hydrogen peroxide and peroxidase derived from tea or horseradish. Kinetic measurements suggest that tea flavanols are generally poorer substrates for peroxidase than for catechol oxidase. However, the level of peroxidase is high and, furthermore, in view of the respective pH optima of the two enzymes, its action would be favoured as the pH falls during fermentation. It is postulated that the nature and distribution of the pigments formed during the fermentation of tea is governed in part by the relative actions of catechol oxidase and peroxidase. These actions are, in turn, influenced by the availability of oxygen and the action of catalase.