A numerical model of the buffer flow in continuous-flow electrophoresis has been developed to study the coupling between transfer phenomena (hydrodynamics, heat transfer and mass transfer) and their role in inducing natural convection that disturbs the required forced convection. The main theoretical unknown in this system is the behaviour of the ion-exchange membranes operating at currents beyond the limiting current. Comparison of the model predictions with experimental observations shows that the traditional water-splitting model is definitely superior to the more recent explanation in terms of electroconvection.