Two pathways for transmembrane reduction of dihexadecylphosphate (DHP) vesicle-entrapped Co(bpy)(3)(3+) ion by photogenerated methyl viologen radical cation have been identified by quantitative evaluation of the reaction kinetics. One pathway involves electrogenic diffusion of MV(+) and the other electroneutral diffusion of MV(0) across the bilayer. The pathways were clearly distinguished by the serf-impeding character of the electrogenic reaction, which was progressively retarded as the membrane polarization increased, and by the net stoichiometry of viologen uptake accompanying transmembrane oxidation-reduction. The first-order rate constants for transmembrane diffusion of MV(+) and MV(0) were estimated to be 2.7 x 10(-2) s(-1) and 1.1 X 10(3) s(-1), respectively, at 23 degrees C. The rate constant for MV(+) diffusion is identical to the value previously measured by C-14-isotopic labeling methods, and : other kinetic parameters were consistent with thermodynamic data obtained from measurements of Donnan equilibria. The relative contributions of the two pathways were dependent upon medium conditions; generally, the electrogenic pathway predominated during the early stages of the reaction but contributed relatively less as the transmembrane potential increased. This behavior explains the difficulties encountered in numerous earlier kinetic studies that were designed to identify the redox carrier.