Cytochrome c3 (cyt c3) can mediate electron transport across phosphatidylcholine (PC)/cardiolipin (CL) and PC/phosphatidylglycerol (PG) membranes. A two-molecule process is involved in the electron transport across PC/CL membranes in the liquid-crystalline state. In contrast, a single-molecule process dominates the electron transport across PC/CL membranes in the gel state and PC/PG membranes in the liquid-crystalline and gel states. Namely, the electron transport mechanism differs with the phospholipid composition and membrane fluidity. The rate-limiting step of the two-molecule process was lateral diffusion of cyt c3 in membranes. The rate constants for the three single-molecule process cases were similar to each other. To elucidate these reaction processes, interactions between cyt c3 and phosphate groups and between cyt c3 and the glycerol backbones of phospholipid bilayers were investigated by means of 31P and 2H solid-state NMR, respectively, for CL and PC/CL membranes. The results showed that the polar headgroups of both phosphatidylcholine and CL are involved in the binding of cyt c3. Also, cyt c3 penetrates into membranes, which would induce distortion of the lipid bilayer. The molecular mechanisms underlying the single- and two-molecule processes are discussed in terms of membrane structure.