The kinetics of oxidation of eight monosubstituted anilines catalyzed by horseradish peroxidase compound II has been studied at pH 7.00 and 7.60. With p-toluidine the rates of oxidation by compound II have been measured at 21 pH values between 3.60 and 10.25. The rate–pH profile indicates that an acidic form of compound II and the electrically neutral, unprotonated form of p-toluidine are reactive. The correlation of rate constants for the substituted anilines with the substituent constant σ in the Hammett equation suggests that the aromatic amine donates an electron to compound II in the rate-controlling step and loses a proton simultaneously. The value of ρ, the susceptibility factor in the Hammett equation, is −6.0 ± 0.7. The reactivity of anilines with HRP-II observed in this study is lower than that of anilines with HRP-I observed previously, although the value of ρ is the same within experimental error (D. Job and H. B. Dunford. Eur. J. Biochem. 66, 607 (1976)). The difference in reactivity is explained by the relative complexities of the reactions of compounds I and II. Keywords: horseradish peroxidase, peroxidase compound II, aniline oxidation, Hammett correlation, enzymatic oxidation.