A chitosan (CS) derivative, N-(4-pyridylmethylidene)-CS (PMC) was synthesized and a cobalt(II) complex of bis(salicylideneethylene diamine) (CoSalen) was immobilized on it. The structure of the immobilized CoSalen was characterized by elemental analysis, IR, XPS, fluorescence and ESR spectrocopy. It was demonstrated that the immobilization of CoSalen was realized through the coordination of nitrogen atom of the pendant group, pyridyl in PMC to the Co(II) in CoSalen. The polymer supported complex is more efficient than the corresponding monomeric complex in catalysing the oxidation of DOPA using oxygen as an oxidizer. The results may be attributed to site isolation effect offered by the supporting polymer chain. A mechanism similar to that for an enzymatic catalysis was proposed.