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A series of seven hemeproteins, cytochromes P-450LM2, P-450LM4, and P-420LM2, horseradish peroxidase, chloroperoxidase, catalase, and metmyoglobin, as well as hemin were tested for their ability to catalyze a set of five oxidative reactions. These reactions were a typical peroxidative reaction (oxidation of pyrogallol to purpurogallin) and three characteristic P-450 reactions (aliphatic hydroxylation, aromatic hydroxylation, and olefinic epoxidation). In addition, the ability to decarboxylate a peroxyacid was measured. All hemeproteins were able to carry out peroxidation, but three (horseradish peroxidase, chloroperoxidase, and catalase) were much better catalysts than the others. Only the P-450 enzymes were competent catalysts for the hydroxylation and epoxidation reactions. Furthermore, the decarboxylation reaction was strictly limited to the P-450 enzymes, establishing it as a new, unique P-450 activity. Since the decarboxylation of peroxyacids is diagnostic of peroxide homolysis, these results indicate a fundamentally different manner of processing of peroxides by cytochrome P-450 than by the peroxidases. Thus, the possibility of close similarity of reactive oxygen intermediates in the two series is called into question.