Electron nuclear double resonance (ENDOR) spectroscopy of oxoiron(IV) porphyrin pi-cation radical complexes with pyrrole beta-substituted porphyrins is reported. Hyperfine coupling constants (A(H)) of the pyrrole beta-methyl groups were determined. Comparison of the A H values with those of compound I of peroxidase and catalase indicated the a(Iu) porphyrin pi-cation radical states of these compound I.