Mitochondrial ubiquinol: cytochrome c reductase (Mr∼600,000) was cleaved into a complex (Mr∼280,000) of the subunits III (cytochrome b), IV (cytochrome c1) and VI to IX, a complex (Mr∼300,000) of the subunits I and II, and the single subunit V (iron-sulphur subunit, Mr∼25,000). Neutron scattering was applied to the whole enzyme, the cytochrome bc1 complex, both in hydrogenated and deuterated alkyl (phenyl) polyoxyethylene detergents, and the complex of subunits I and II in detergent-free solution. The neutron parameters were compared with the structures of the enzyme and the cytochrome bc1 complex previously determined by electron microscopy. Using the method of hard spheres, comparison of the calculated and experimental radius of gyration implies that the length of the enzyme across the bilayer or the detergent micelle is between 150 and 175 Å and of the cytochrome bc1 complex between 90 and 115 Å. The subunit topography was confirmed. The cleavage, plane between the cytochrome bc1 complex and the complex of subunits I and II lies at the centre of the enzyme and runs parallel to the membrane just outside the bilayer. The detergent uniformly surrounds the protein as a belt, which is displaced by 30 to 40 Å from the protein centre of the enzyme and by about 20 Å from the protein centre of the cytochrome bc1 complex. The low protein matchpoint of the whole enzyme as compared to the subunit complexes is accounted for in terms of the non-exchange of about 30 to 60% of the exchangeable protons within the intact enzyme. Polar residues are, on average, at the protein surface and non-polar residues and polar residues with non-exchanged protons are buried within the enzyme. © 1983 Academic Press Inc. (London) Ltd.