The kinetics of p-aminobenzoic acid oxidation catalyzed by horseradish peroxidase Compounds I and II was investigated intensively as a function of pH at 25 degrees in aqueous solutions of ionic strength 0.11. All of the rate data were collected from single turnover experiments involving reactions of a single enzyme compound. In reactions of both compounds, deviations from first order behavior with respect to the enzyme were observed at high pH values which were explained in terms of a free radical interaction of product with the enzyme. The effect could be eliminated with sufficient excess of substrate. Kinetic behavior which deviated from first order in substrate, observed at low pH, was explained by a mechanism involving an enzyme-substrate complex which reacted with an additional molecule of substrate but at a slower rate. The pH dependence of the second order rate constants for the reaction of p-aminobenzoic acid with free Compounds I and II is similar to results obtained for the comparable reactions of ferrocyanide, suggesting similar proton-transfer mechanisms for both reducing substrates. The reduction of Compound II by p-aminobenzoic acid appeared to be influenced by two ionizable groups on the enzyme which affect the electronic environment of the heme. The lack of influence of substrate ionizable groups on the rate of the Compound II reaction indicated that potential differences in reactivities of NH2C6H4COO- and NH2C6H4COOH were levelled by the diffusion-controlled limit in the acid region of pH. The reduction of Compound I by p-aminobenzoic acid was not diffusion-controlled and the rate-pH profile could be explained in terms of three acid ionizations, two on the substrate and one on Compound I.