Treatment of the mono- meso-substituted iron(II) octaethylporphyrin complexes, (py)(2)Fe-II(meso-NO2-OEP). (py)(2)Fe-II(meso-CN-OEP), (py)(2)Fe-II(meso-HC(O)-OEP) (py)(2)Fe-II(meso-Cl-OEP), (py)(2)Fe-II(meso-OMe-OEP), (py)(2)Fe-II(meso-Ph-OEP), and (py)(2)Fe-II(meso-n-Bu-OEP), with hydrogen peroxide in pyridine-d(5) at -30 degreesC in the strict absence of dioxygen has been monitored by H-1 NMR spectroscopy. The product oxophlorin complexes are stable as long as the samples are protected from exposure to dioxygen. Hydrogen peroxide reacts cleanly with mono-meso-substituted iron(II) porphyrins in pyridine solution under an inert atmosphere to form mixtures of three possible oxygenation products, (py)(2)Fe(cis-meso-R-OEPO), (py)(2)Fe(trans-meso-R-OEPO), and (py)(2)Fe(OEPC). The yields of (py)(2)Fe(OEPO), which results from replacemement of the unique meso substituent, as a function of the identity of the meso substituent decrease in the order NO2 > HC(O) similar to CN similar to Cl > OMe > Ph, Bu, which suggests that the species responsible for attack on the porphyrin periphery is nucleophilic in nature. A mechanism involving isoporphyrin formation through attack of hydroxide ion on a cationic iron porphyrin with an oxidized porphyrin ring is suggested. The identity of the unique meso functionality also affects the regiospecificity of substitution when the unique meso group is retained. Although random attack at the two different meso sites is expected to yield a cis/trans product ratio of 2, the observed ratios vary in the following order: cyano, 5.0; n-butyl, 4.9; chloro, 3.2; formyl, 2.6: methoxy, 1.9; phenyl 1.4.