The kinetics of the decomposition of H 2 O 2 catalyzed by Cu(II) has been studied by the initial-rate method in aqueous phosphate media at near physiological pH. The activity of the catalyst is increased by [Fe(CN) 6 ] 3 - and decreased by VO 3 - , CrO 4 2 - and Zn(II). Three reaction pathways are involved in the Cu(II)-H 2 O 2 reaction, the kinetic orders of the catalyst being 1 (rate constant k 1 ), 2 (rate constant k 2 ) and 3 (rate constant k 3 ). The three pathways present fractional apparent orders (>1) in H 2 O 2 and base catalysis. The apparent activation energies associated to rate constants k 1 , k 2 and k 3 are 102+/-4, 65+/-8 and 61+/-5 kJ mol - 1 . Free-radical chain mechanisms are proposed for the three pathways.