The reaction of [Fe-II(tpa)(OTf)(2)] (tpa = tris(2-pyridylmethyl)amine) and its related 5-Me-3-tpa complex with hydrogen peroxide affords spectroscopically distinct iron(III)peroxo intermediates in CH3CN and acetone. The reaction in acetonitrile at -40degreesC results in the formation of the previously reported Fe-III-OOH intermediate, the end-on hydroperoxo coordination mode of which is established in this paper by detailed resonance Raman isotope-labeling experiments. On the other hand, the reaction in acetone below -40degreesC leads to the observation of a different peroxo intermediate identified by resonance Raman spectroscopy to be an Fe-III-OOC (CH3)(2)OH species; this represents the first example of an intermediate derived from the adduct of H2O2 and acetone. The peroxoacetone intermediate decays more rapidly than the corresponding Fe-III-OOH species and converts to an Fe-IV-O species by O-O bond homolysis. This decay process is analogous to that observed for [Fe-III(tpa)(OOtBu)](2+) and in fact exhibits a comparable enthalpy of activation of 54(3) kJ mol(-1). Thus, with respect to their physical properties at low temperature, the peroxoacetone intermediate resembles [Fe-III(tpa)(OOtBu)](2+) more than the corresponding Fe-III-OOH species. At room temperature, however, the behavior of the Fe(tpa)/H2O2 combination in acetone in catalytic hydrocarbon oxidations differs significantly from that of the Fe(tpa)/tBuOOH combination and more closely matches that of the Fe(tpa)/H2O2 combination in CH3CN. Like the latter, the Fe(tpa)/H2O2 combination in acetone catalyzes the hydroxylation of cis-1,2-dimethylcyclohexane to its tertiary alcohol with high stereoselectivity and carries out the epoxidation and cis-dihydroxylation of olefins. These results demonstrate the subtle complexity of the Fe(tpa)/H2O2 reaction surface.