Hemin coordinated with mercaptide sulfur as fifth ligand and various sixth ligands were investigated as models for cytochrome P450 in its native ferric low-spin state and its ligand complexes. Mixing the hemin with its ligands below -60 degrees C prevented the reduction of the hemin by mercaptide and made it possible to characterize each sample both by electronic and ESR spectra. Excess of mercaptide formed hemin-dimercaptide complexes with hyperporphyrin spectra with two Soret bands around 380 and 370 nm. The second mercaptide could be exchanged by other ligands with hydroxyl, phosphine, thioether, isocyanide, amine, imidazole, and pyridine groups. The comparison of these spectral data with cytochrome P450 substantiates mercaptide as the fifth ligand and makes a hydroxyl group a more likely candidate for the native sixth ligand than an imidazole group.