Mössbauer spectroscopy has been used to study the cytochromes cc′ (RHP) and c-type cytochromes from the photosynthetic bacteria Rhodospiril-lum rubrum and Chromatium. The data for the proteins enriched in 57Fe show that the two heme groups in the cytochromes cc′ must be similar in iron coordination. The oxidized cytochromes cc′ have a highly distorted high-spin electronic configuration, like that observed for methemoglobin. However, the signs of the electric field gradients are apparently opposite for methemoglobin and for the cytochromes cc′, indicating that the iron coordination is very different for the two proteins. The “thermally mixed” spin states attributed to these proteins are not observable as independent entities over the time (10-8 sec) characteristic of a Mössbauer measurement, but may be manifest in the temperature dependence of the quad-rupole splitting. The cytochrome C552 from Chromatium has Mössbauer spectral characteristics very similar to those of the c-type cytochrome from R. rubrum, despite dissimilarities in redox potential, size, and CO binding ability. The two heme groups of the cytochrome C552 also appear identical in iron coordination. The cytochromes cc′ are high-spin Fe2+ molecules in the reduced state, in contrast to the diamagnetic state of the R. rubrum cytochrome c2 or the Chromatium cytochrome c552. This fundamental difference in iron electronic configuration is another basis for considering the cytochromes cc′ as a class distinct from c-type cytochromes. © 1968, American Chemical Society. All rights reserved.