The N-alkylhemin formation during iron(III) porphyrin-catalyzed epoxidation of alkenes is studied in a homogeneous system using pentafluoroiodosobenzene (PFIB) as the oxidant. The system is a model for the enzyme cytochrome P-450. A second N-alkyl species other than the well-documented primary N-alkylhemin is observed during the oxidation of terminal alkenes. It is unstable and decomposes under the reaction conditions. It is also a catalyst for alkene epoxidation by PFIB. Its characteristics are consistent with a secondary N-alkylhemin. The kinetics were partially resolved, and rate constants-were estimated for the formation of both N-alkylhemins, for the decomposition of the secondary N-alkylhemin, and for the epoxidation by the hemin and the N-alkylhemins. The partition ratios were measured by a kinetic method, which provides better insight for the mechanisms of both N-alkylation and epoxidation than the measurements by product analysis using heterogeneous systems. We suggest that alkenes reversibly inhibit cytochrome P-450 through the formation of the unstable N-alkylporphyrin species.