A cofactor having an aromatic or benzylic amino group is converted into an isothiocyanate by reaction with a compound such as thiophosgene and the isothiocyanate of the cofactor is attached to a polymer which is preferably water-soluble. When the polymer has amino groups, a thiourea bridge is formed between the polymer and cofactor and a polymer having hydroxyl groups results in a thiocarbamate bridge. The polymer may be a copolymer of vinylamine or vinylmethylamine and vinylmethylacetamide, partially alkylamine-substituted .alpha., .beta.-poly-(2-hydroxyethyl)-D,L-aspartamide, polyethyleneimine or carbohydrate. The cofactor bound to a polymer and an enzyme are contained in microcapsules or in a membrane to form an enzyme reactor.