Membrane potential differences of a polymethacrylate–polyacrylamide snake-cage membrane were measured at 25°C. The principal theories of membrane potential have been tested. The fixed-charge theory is only valid in a range of high membrane permselectivity. The relation developed by the kinetic approach and the thermodynamics of irreversible processes verify the experimental results, but the calculation of some parameters, often improperly assumed to be constant, is not unquestionable. Quasithermodynamics (Scatchard's relation) probably give the best estimates of the membrane potential on the basis of the independent values of transport numbers of counterion and water.