Near-quantitative formation of an oxoiron(IV) intermediate [Fe-IV(O)(TMC)(CH3CN)](2+), (2) from stoichiometric H2O2 was achieved with [Fe-II(TMC)](2+) (1) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane). This important outcome is best rationalized by invoking a direct reaction between 1 and H2O2 followed by a heterolytic O-O bond cleavage facilitated by an acid-base catalyst (2,6-lutidine in our case). A sizable H/D KIE of 3.7 was observed for the formation of 2, emphasizing the importance of proton transfer in the cleavage step. Pyridines with different pK(a) values were also investigated, and less basic pyridines were found to function less effectively than 2,6-lutidine. This study demonstrates that the reaction of Fe(II) with H2O2 to form Fe(IV)=O can be quite facile. Two factors promote the near-stoichiometric conversion of H2O2 to Fe(IV)=O in this case: (a) the low reactivity between 1 and 2 and (b) the poor H-atom abstracting ability of 2, which inhibits subsequent reaction with residual H2O2. Both factors inhibit formation of the Fe(III) byproduct commonly found in reactions of Fe(II) complexes with H2O2. These results may shed light into the nature of the O-O bond cleaving step in the activation of dioxygen by nonheme iron enzymes and in the first step of the Fenton reaction.