One method of synthesizing new materials is to react functional pendant groups on an existing polymer. Here, an enzymatic strategy was investigated as a method of producing new polymers from chitosan. The enzyme used was tyrosinase which is known to oxidize a range of phenolic reactants, yielding reactive o-quinones. The products of this enzymatic oxidation appear to react with, and alter the chemical properties of chitosan. Specifically, it was observed that when thin chitosan films were treated with both tyrosinase and a phenolic reactant, the u.v.-visible spectra and the acid-base properties were markedly altered compared with films that were untreated.