A process for oxidising a substrate selected from hydroxymethylfurfural (HMF), diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA) and formylfurancarboxylic acid (FFCA). Said process comprises mixing said substrate with catalase, one or more further enzymes and hydrogen peroxide to form a reaction mixture. Said one or more further enzymes have the ability to catalyse oxidation of said substrate. Said hydrogen peroxide is provided at a total molar ratio of at least about 0.1:1 hydrogen peroxide to substrate.