Abstract The flash-induced formation of transmembrane electric potential differences (measured by carotenoid bandshift) and redox changes of cytochrome b h ( b 561) were monitored spectrophotometrically in Rb. sphaeroides chromatophores in a pH range from 7.5 to 10.0. It is shown that in the presence of antimycin A and at pH < 8.3 the myxothiazol-sensitive, antimycin-insensitive component of the carotenoid bandshift is kinetically coupled to cytochrome b h reduction. The kinetics of both processes can be described by a single exponent with a rise time of about 10 ms. Alkalization of the medium (8.3 ≤ pH ≤ 9.2) causes the appearance of an additional constituent in this phase of the carotenoid response with the rise time varying in the range of 100–300 ms. With a further pH increase (pH > 9.2), the electrogenic constituent, kinetically linked to cytochrome b h reduction, diminishes. The obtained data are discussed within the framework of the scheme, assuming that the electron transfer between b l and b h hemes in the bc 1 complex is, under certain conditions, accompanied by proton transfer in the same direction.