1. When cytochrome c2 is available for oxidation by the photosynthetic reaction centre, the decay of the carotenoid absorption band shift generated by a short flash excitation of Rhodopseudomonas capsulata chromatophores is very slow (half-time approximately 10 s). Otherwise the decay is fast (half-time approximately 1 s in the absence and 0.05 s in the presence of 1,10-ortho-phenanthroline) and coincides with the photosynthetic back reaction. 2. In each of these situations the carotenoid shift decay, but not electron transport, may be accelerated by ioniophores. The ionophore concentration dependence suggests that in each case the carotenoid response is due to a delocalised membrane potential which may be dissipated either by the electronic back reaction or by electrophoretic ion flux. 3. At high redox potentials, where cytochrome c2 is unavailable for photooxidation, electron transport is believed to proceed only across part of the membrane dielectric. Under such conditions it is shown that the driving force for carbonyl cyanide trifluoromethoxyphenyl hydrazone-mediated H+ efflux is nevertheless decreased by valinomycin/K+; demonstrating that the [BChl]2 leads to Q electron transfer generates a delocalised membrane potential.