The iron complexes [(BPMEN)Fe(OTf)(2)] (1) and [(TPA)Fe(OTf)(2)] (2) [BPMEN = N,N '-bis-(2-pyridylmethyl)-N,N '-dimethyl-1,2-ethylenediamine; TPA = tris-(2-pyridylmethyl)amine] catalyze the oxidation of olefins by H2O2 to yield epoxides and cis-diols. The addition of acetic acid inhibits olefin cis-dihydroxylation and enhances epoxidation for both 1 and 2. Reactions carried out at 0 degrees C with 0.5 mol % catalyst and a 1:1.5 olefin/H2O2 ratio in a 1:2 CH3CN/CH3COOH solvent mixture result in nearly quantitative conversions of cyclooctene to epoxide within 1 min. The nature of the active species formed in the presence of acetic acid has been probed at low temperature. For 2, in the absence of substrate, [(TPA) FeIII(OOH)(CH3COOH)](2+) and [(TPA) (FeO)-O-IV(NCCH3)](2+) intermediates can be observed. However, neither is the active epoxidizing species. In fact, [(TPA) (FeO)-O-IV(NCCH3)](2+) is shown to form in competition with substrate oxidation. Consequently, it is proposed that epoxidation is mediated by [(TPA Fe-V(O)(OOCCH3)](2+), generated from O-O bond heterolysis of the [(TPA) Fe-III(OOH) (CH3COOH)](2+) intermediate, which is promoted by the protonation of the terminal oxygen atom of the hydroperoxide by the coordinated carboxylic acid.