The pH dependence of the peroxidase activity (guaiacol assay) of the ferric hemeoctapeptide N-acetylmicroperoxidase-8 (N-AcMP8) was studied under conditions where formation of the Compound I analogue of the peroxidase enzymes is rate limiting. The pH profile of the reaction rate is consistent with a mechanism where both H2O2 and HO2- can displace H2O coordinated trans to neutral His but where the hydroxo complex of the hemepeptide (OH- trans to His) is kinetically inert. At pH > 11, where the proximal His ligand of Fe(III) ionizes to form a histidinate, the hydroxo complex (OH- trans to His-) becomes kinetically labile. A comparison of Delta H-double dagger and AS(double dagger) values for the reaction of (HO2)-O-2 and HO2-, obtained from the temperature dependence of the rate constants, with values for CN-and cysteine reported previously, shows that the activation parameters depend on the identity of the incoming ligand. This suggests that ligand substitution at Fe(III) in N-AcMP8 proceeds through an interchange mechanism.