Oxoiron(IV) porphyrins were conveniently prepared by ligand-metathesis of stable iron(III) porphyrin cation radicals over moist basic alumina. The formation of oxoiron(IV) porphyrins by electrochemical oxidation of the corresponding hydroxoiron(III) porphyrin was shown to be an EC process, with initial oxidation of the porphyrin ring followed by elimination of HClO4 and rearrangement. Oxo(tetramesitylporphyrinato)iron(IV), (TMP)Fe-IV = O (1), was found to be stable at room temperature in relatively unreactive solvents such as benzene. (TMP)-Fe-IV = O (1) reacted readily with substituted styrenes with a selectivity which was very much unlike that observed for (TMP(+))Fe-IV = O (4). Kinetic data and product analyses in benzene indicate that the first and rate limiting step is attack of the ferryl oxygen of 1 on the olefin pi-bond to form a carbon radical intermediate (5), followed by a fast reaction with molecular oxygen to form benzaldehydes or ring closure to afford epoxides with loss of the configuration of the starting olefin. In more polar solvents the results indicated a disproportionatation of 1 to afford 4 as the reactive intermediate.