A systematic study of the electronic structure of models for the active sites of heme enzymes such as peroxidases and cytochromes P450 has been carried out for high-valent transition states of their catalytic cycles, namely, compound I and compound II for peroxidases, as a function of the second axial ligand. The investigation is based on molecular orbital calculations in local density approximation and comprises five-coordinated oxoiron-(IV) porphyrin as well as the corresponding six-coordinated species with chlorine, imidazole, and H3CS- as axial ligands. In all cases, the ground state of compound II is obtained as the ferryl (t(2g))(4) configuration (3d(xy))(2)(3d(xz),3d(xy))(2) with total spin S = 1 distributed between the iron and oxygen atom in a ratio of approximately 60/40. Different electronic states of compound I with the radical located in the a(1u) or a(2u) orbitals of the porphyrin or in the lone-pair orbital of the axial ligand are discussed in detail. The corresponding Heisenberg exchange coupling constants J between the oxoiron and the radical spin are calculated, and the influence of the position and orientation of the axial ligand on J is investigated. The results are correlated with the available experimental data.