The decomposition reaction of hydrogen peroxide by a water-soluble polymer containing phthalocyanine-Mn (1c, 1d) was studied in aqueous solution. The catalase-like activity was varied by the kind of added nitrogen base and its concentration. The catalase-like activity is also dependent upon the basicity of the nitrogen base and the equilibrium constant for the coordination of the nitrogen base to phthalocyanine-Mn in 1c or 1d. In this catalytic system, the deprotonation of hydrogen peroxide and the formation of a five-coordinated manganese species are an important factor in the catalase-like activity. Copolymers 1c and 1d have high activity as catalyst for the decomposition of hydrogen peroxide in the presence of dimethylaminopyridine (DMAPy). The catalytic system obeys Michaelis-Menten kinetics with respect to the concentration of hydrogen peroxide. The reaction proceeds according to a mechanism similar to that of a reaction catalyzed by catalase.