The transfer rates of Na+ , K+, Mg2+ and Ca2+ across a cation exchange membrane were investigated. Two models were examined to explain the experimental results. The first model is based on the ion exchange equilibrium at the membrane surface, the second on the nonequilibrium exchange kinetics at the membrane surface and within the membrane. The transfer rates calculated by the second model were in good agreement with the experimental results for both monovalent and divalent ions.