The kinetics of electron transfer through vesicle membranes mediated by viologen was considered in detail. In previous studies, the rate-determining step of the reaction has been found to be the disproportionation of two viologen radicals : 2C(n)V(+) double left right arrow CnV2+ + CnV0. The effect of reactant compartmentalization was demonstrated by both experiments and simulations. Second-order fits to the kinetic data showed that disregarding compartmentalization is a good approximation for the major part of the observed reaction when the initial average occupancy number [n(o)] greater than or equal to 10. The value for the rate constant obtained from a "good" fit was unaffected by compartmentalization. Both simulations and experiments showed that when [n(o)] less than or equal to 5, it was not possible to obtain a satisfactory second-order fit over the same interval. Experiments showed that migration of CnV2+ between vesicles was rapid (1/k(-) < 1 ms), but migration of CnV+ was slow on the time scale of the experiments. The effect of viologen migration was demonstrated by simulations. The simulation results are general for compartmentalized reactions described by 2A --> A + B, with and without the possibility of intercompartment exchange of A during the reaction.