The purified cytochrome b-c1 complex of Rhodopseudomonas sphaeroides has two b cytochromes distinguishable by optical, thermodynamic and electron paramagnetic resonance criteria (gz values are approximately equal to 3.75 and approximately equal to 3.4). EPR features typical of a Rieske iron sulfur cluster (g values of 2.03 1.90 and 1.81) and a c1 type cytochrome (g approximately equal to 3.4) were also observed. The b and c1 cytochromes were individually purified from the complex. The cytochrome c1 retained its native EPR spectrum. The b cytochrome lost over 90% of the intensity from the 'b566 type' heme site (g approximately equal to 3.75), while the 'b561 type' heme site (g approximately equal to 3.4) retained its native EPR spectrum.