Two new models for cytochrome P 450 in which the thiolate axial ligand is replaced by a RSO3- group, form oxo-iron (IV) porphyrin pi-cation radicals as sole oxidn. products in "peroxo shunt" reactions independent of the nature of the employed solvent (polar or non-polar) and electronic nature of the porphyrin rings.  Although the properties of the solvent and push-pull effects from the porphyrin rings do not affect the mode of the O-O bond cleavage (heterolytic or homolytic) in these models, they strongly affect the rate and mechanism of each reaction step leading to the formation of the high-valent iron intermediates.  This article reports the results of mechanistic studies involving the measurements of the rate of oxo-iron (IV) porphyrin pi-cation radical formation from the enzyme mimics of P 450 for different oxidant concn., temp. and pressure in selected org. solvents.  Extn. of the appropriate rate consts. and activation parameters for the reactions studied enable a detailed discussion of the effects of solvent and electronic nature of the porphyrin rings on the position of the first pre-equil. involving formation of the acylperoxo-iron (III) porphyrin intermediate, as well as on the rate of heterolytic O-O bond cleavage leading to the formation of the high-valent iron species.  Furthermore, an unusual effect of solvent on the kinetics of oxo-iron (IV) porphyrin pi-cation radical formation in methanol is demonstrated and discussed in the present work.