Mossbauer spectra are reported for Chromatium ferricytochrome c′ in the pH range 1.0-10.5. Measurements were made in the 150°-4.2°K range in the absence of an external field, and at 4.2°K in longitudinal and transverse applied fields. Samples at pH 1.0 and 10.5, 150°K, show a strongly asymmetric, diffuse doublet, similar to hemin. Spectra of the protein at pH 1.0 and 10.5, 4.2°K, exhibit the 6-line hyperfine pattern typical of high-spin ferric heme proteins, consistent with previous EPR data. Protein samples at neutral pH and 150°K show a clear symmetric doublet with a quadrupole splitting of magnitude 2.31 mm/s (±0.02 mm/s) and an isomer shift of 0.35 mm/s relative to the center of an Fe metal absorption spectrum. At neutral pH and 4.2±K, the spectrum consists of a 4-line hyperfine pattern which is substantially different from the high and low pH forms or published spectra of high-spin or low-spin ferric heme proteins. The spectral transitions are consistent with interpretations of previous EPR data. The neutral pH hyperfine pattern is characteristic of a species having a large A⊥, a small A∥, and a quadrupole interaction whose strength is comparable with the strength of the magnetic hyperfine interaction. The 4-line hyperfine spectrum was computer fitted to a quantum mixed-spin model, yielding values of 2.91 mm/s for the quadrupole splitting, 0.29 mm/s for the isomer shift, and 6.2 T per electronic spin for the Fermi contact term. Implications of this unusually low value are discussed. © 1980 American Institute of Physics.