Iron(III)-salen complexes catalyze the H 2 O 2 oxidation of various ring-substituted anilines in MeCN have been studied, and [O=Fe IV (salen)] ?Á is proposed as the active species. Study of the kinetics of the reaction by spectrophotometry shows the emergence of a new peak at 445 nm in the spectrum which corresponds to azobenzene. Further oxidation of azobenzene by H 2 O 2 leads to the formation of azoxybenzene. ESI-MS studies also support the formation of these products. The rate constants for the oxidation of meta-and para-substituted anilines were determined from the rate of decay of oxidant as well as the rate of formation of azobenzene, and the reaction follows Michaelis-Menten kinetics. The rate data show a linear relationship with the Hammett r constants and yield a q value of -1.1 to -2.4 for substituent variation in the anilines. A reaction mechanism involving electron transfer from aniline to [O=Fe(salen)] ?Á is proposed. The presence of axial ligands modulates the activity of the complex.Electronic supplementary material The online version of this article (