The soluble domains of the b and b′ subunits of the ATP synthase of the cyanobacterium Synechocystis PCC 6803 were expressed with His tags attached to their N-termini. Following purification, the polypeptides were characterized by chemical cross-linking, analytical ultracentrifugation, and circular dichroism spectroscopy. Treatment of a mixture of the soluble b and b′ domains with a chemical cross-linking agent led to substantial formation of cross-linked dimers, whereas similar treatment of either domain by itself resulted in only trace formation of cross-linked species. The molecular weights of the domains of b and b′ in solution at 20 °C, measured by sedimentation equilibrium, were 17 800 ± 700 and 16 300 ± 400, respectively, compared to calculated polypeptide molecular weights of 16 635 and 15 422, whereas a mixture of b and b′ gave a molecular weight of 29 800 ± 800. The sedimentation coefficient of an equimolar mixture was 1.73 ± 0.03. The circular dichroism spectra of the individual polypeptides indicated helical contents in the range of 40-50%; the spectrum of the mixture revealed changes indicative of coiled-coil formation and a helical content of 60%. The results indicate that the cytosolic domains of the b and b′ subunits exist individually as monomers but form a highly extended heterodimer when they are mixed together.