Cytochrome c′ from Rhodospirillum rubrum has been investigated in the ferric form with Mössbauer and EPR spectroscopy. In the pH range from 6 to 9.5, three species are observed which belong to two pH-dependent equilibria with pK values near 6 and 8.5. The pK 6 transition is resolved only with high-field Mössbauer spectroscopy. For the three species we have determined the zero-field splitting parameters and the hyperfine coupling constants. The data were fitted to a spin Hamiltonian which takes into account a weak mixing of excited S 3 2 states into the sextet ground manifold. The low temperature spectra clearly show that the quadruple coupling constant δEo is positive for ferricytochrome c′ and thus in accord with all other high-spin ferric heme proteins.