Electro-osmotic fluxes of water through two cation-exchange membranes have been measured under different experimental conditions. The influence of unstirred solution layers adjoining the membrane surfaces on the obtained values has been studied both theoretically and experimentally A model has been developed which gives quantitative information about the concentration polarization effects on the electro-osmotic fluxes. The theoretical predictions are compared to the experimental results, and the accordance may be considered satisfactory.