A stopped-flow kinetic study shows that the reaction rates of lactoperoxidase compound II with substituted phenols are greatly dependent upon the substituent on the benzene ring. Of the 15 phenols studied, it has been possible to relate the reaction rate constants with the ionization constants of 11 phenols by a linear Hammett free-energy relationship: (log (kX/kH) = ρσ) at pH 7.0 and 25.0 °C. The 11 m-and p-substituted phenols yield a ρ value of −2.7 ± 0.2. The data do not correlate with σ+ values using the Brown–Okamoto equation. These results can be explained by a mechanism in which the substrate simultaneously gives an electron and a proton to lactoperoxidase compound II, and they indicate that the active site is accessible to substrate. To explain the abnormal behavior of the four phenols that do not fit the Hammett plot we used Hansch hydrophobic factors π for all of the substituents. The resultant best-fit equation: log (kX/kH) = ρσ + rπ + c yielded the values ρ = −2.1 ± 0.5, r = 0.4 ± 0.1, and c = −0.2 ± 0.1. The rates of reduction of lactoperoxidase compound II by p-iodophenol have been studied from pH 6.5 to 11.2. The kinetics are influenced by an acid group of pKE 9.5 in the active site of lactoperoxidase compound II and by the ionization of the substrate (pKS 9.0). The acidic form of lactoperoxidase compound II and the electrically neutral, unionized form of phenol are the reactive species. A reanalysis of data on reactions of horseradish peroxidase compounds I and II with substituted phenols using more recent σ values yielded Hammett ρ values of −5.5 ± 0.3 and −4.5 ± 0.2.