AbstractAddition of [FeII(MeCN)24+(ClO−4)2to solutions of hydrogen peroxide in dry acetonitrile (MeCN) catalyzes a rapid disproportionation of H2O2via the initial formation of an adduct, [FeII(HOOH)↔Fe(O)(OH2)]2+, which oxidizes a second H2O2to dioxygen. This intermediate also cleanly oxidizes substituted hydrazines, alcohols, aldehydes, and thioethers by a two‐electron process. The products for these H2O2oxidations are consistent with those that result from catalase‐ and some peroxidase‐catalyzed processes. In the same aprotic medium (MeCN) anhydrous FeIIICl3catalyzes the demethylation ofN,N‐dimethylaniline, the epoxidation of olefins, and the oxidative cleavage of 1‐phenyl‐1,2‐ethanediol (and other 1,2‐diols) by hydrogen peroxide. A mechanism is proposed in which an initial Lewis acid‐base interaction of FeIIICl3with H2O2generates a highly electrophilic FeIII‐oxene species as the reactive intermediate. For each class of substrate the products closely parallel those that result from their enzymatic oxidation by cytochrome P‐450. Because of (a) the close congruence of products, (b) the catalytic nature of the FeIIICl3/H2O2reaction mimic, and (c) the similarity of the dipolar aprotic solvent (acetonitrile) to the proteinaceous lipid matrix of the biomembrane, the form of the reactive intermediate may be the same in each case. This is in contrast to the prevailing view that cytochrome P‐450 acts as a redox catalyst to generate an Fe(V)‐oxo species or an Fe(IV)‐oxo cation radic