Competitive epoxidation of norbornylene and alpha-methylstyrene by peracids in the presence of iron porphyrins has been carried out in methylene chloride and toluene at -78 degrees C. When (TMP)Fe-III(RCO(2)) (TMP = 5,10,15,20-tetramesitylporphyrin) is used as a catalyst in methylene chloride, selectivity of norbornylene oxide over alpha-methylstyrene oxide is almost identical even though a variety of peracids are used, since O=Fe(IV)TMP pi-cation radical 2a is the common oxidant in the reactions. On the other hand, the selectivity in the epoxidations catalyzed by (TDCPP)Fe-III(X) (TDCPP : 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin is found to be dependent on the structure of peracids. The latter type of the results is also obtained for the reactions catalyzed both by (TMP)Fe-III(X) and by (TDCPP)Fe-III(X) in toluene. The dependence of the selectivity in the epoxidation on the structure of peracids is attributed to the alteration of the oxidant from 2 to peracid-Fe(III) complexes 1. The direct involvement of 1 in the epoxidation has been further confirmed by spectroscopic studies of the reactions. Under the catalytic epoxidation conditions at low temperature, the participation of 1 and 2 as the active species is controlled either by electronic structure of iron porphyrin complexes (push-pull effect) or by alteration of the solvent.