Succinate-cytochrome c reductase can be easily solubilized in a phospholipid mixture (1:1, iysolecithin:lecithin) in the absence of detergents. The resulting solution contains two b cytochromes with half-reduction potentials of 95 plus or minus 10 mV (b561), and 0 plus or minus mV (566) and cytochrome c1 (Em7.2 equals +280 plus or minus 5 mV). The oxidation-reduction midpoint potentials obtained by optical potentiometric titrations are identical to those determined by the EPR titrations and are 40-60 mV higher than the corresponding midpoint potentials of these cytochromes in intact mitochondria. In contrast to detergent-suspended preparations, no CO-sensitive cytochrome b can be detected in the phospholipid-solubilized preparation or intact mitochondria. The half-reduction potential of cytochrome b566 is pH-dependent above pH 7.0 ( minus 60 mV/pH unit) while that of b561 is essentially pH-independent from pH 6.7-8.5, in contrast to its pH dependence in intact mitochondria. EPR characterizations show the presence of three oxidized low-spin heme-iron signals with g values of 3.78, 3.41 and 3.37. The identification of these signals with cytochromes b566(bT), b561 (bK) and c1 respectively is made on the basis of redox midpoint potentials. In addition, the preparation contains four distinct types of iron-sulfur centers: S1 and S2 (Em7.4 equals minus 260 mV and 0 mV), and two iron-sulfur proteins which are associated with the cytochrome b-c1 complex: Rieske's iron-sulfur protein (Em7.4 equals +280 mV) and Ohniski's Center 5 (Em7.4 equals +35 mV).