We report evidence for the existence of distinct active oxidizing species in a cytochrome P-450 model system. Competitive oxidations of alkane substrates were carried out using [5,10,15,20-tetrakis-(pentafluorophenyl)porphyrinato]iron(III) chloride (Fe(TPF5P)Cl) with various iodosylarenes as the oxygen source. The oxidation rate ratios for each pair of substrates were found to differ when different iodosylarenes were employed as terminal oxidants. These competitive oxidation results unambiguously demonstrate that the active oxidants generated from the various terminal oxidants are not the same. The influence of the oxygen donor on the selectivity requires that the donor is involved in the product-determining step. We propose that the active oxidant is a complex between the catalyst and the terminal oxidant.