High-valent iron(IV) oxo porphyrin cation radical complexes containing a series of substituents at the meso position of the porphyrin ring (i.e., electron-donating and -withdrawing substituents on phenyl groups) were prepared and used in oxygen atom transfer reactions to elucidate the electronic effect of porphyrin ligands on the reactivities of iron porphyrin complexes. The reactions that we studied with the in situ generated high-valent iron oxo porphyrins were (1) the relative reactivities of the intermediates toward oxygen atom transfer and ROOH disproportionation (ROOH = hydrogen peroxide and tert-butyl hydroperoxide), (2) the mechanism of heterolytic versus homolytic O-O bond cleavage of hydroperoxides, (3) the dependence of oxidizing power of the intermediates on the electronic nature of porphyrin ligands, and (4) the relative rates between oxygen atom transfer and oxygen exchange with labeled (H2O)-O-18. We found from these reactivity studies that (I) a high-valent iron oxo porphyrin complex containing electron-donating substituents reacts fast with ROOH in a competitive reaction performed with a mixture of olefin and ROOH, whereas a high-valent iron oxo porphyrin containing electron-withdrawing substituents transfers its oxygen atom to olefin to give an epoxide product at a fast rate, (2) the O-O bond of hydroperoxides is homolytically cleaved by iron porphyrin complexes in aprotic solvent, (3) a high-valent iron oxo complex of electron-deficient porphyrin ligand is a more powerful oxidizing species than that of electron-rich porphyrin ligand in alkane hydroxylation reactions, and (4) the presence of electron-donating substituents on a porphyrin ligand gives a relatively high O-18 incorporation from labeled (H2O)-O-18 into an oxygenated product when a mixture of olefin and (H2O)-O-18 is added to a reaction solution containing a high-valent iron oxo intermediate, whereas only a small amount of O-18 incorporation is observed with iron porphyrin complexes containing electron-withdrawing substituents. These results clearly demonstrate that the electronic nature of iron porphyrin complexes is an important factor in determining the reactivities of iron porphyrin complexes in oxygen atom transfer reactions.