A study was conducted to demonstrate olefin-dependent discrimination of two nonheme HO-Fev=O tautomeric species in catalytic H2O 2 epoxidations. Mechanistic studies were carried out under the condition of excess of olefin to minimize over-oxidation reactions and all reactions for the study were carried out under a N2 atmosphere to prevent auto-oxidation process due to presence of O2. It was observed that the diol/epoxide (D/E) ration for these reaction was dependent on the specific olefin and ranged from 3/2 (cyclooctene) to 6/1 (1-octene). The oxidation of cyclooctene using H218O2 revealed that only 28% of the oxygen atoms in the epoxide derived from H 2O2. Mechanistic results suggested that HO-Fe v=O oxidant need to be labeled before its reaction with substrates.