Pervaporation of a water-ethanol mixture with sulphonated polyethylene membranes of various capacities at various temperatures and for various feed compositions was studied both theoretically and experimentally. A model is proposed that accounts both for the effects of partial immobilization of water in the membrane due to adsorption by the groups and for the obstruction effect of the polymer matrix on the basis of percolation arguments. Analysis of the pervaporation data against the results of previous sorption experiments showed that the model is in fair agreement with the experiment. A number of peculiarities not predicted by the model and not observed for ethanol transport, such as enhanced diffusivity and selectivity and reduced apparent activation energy, were observed concerning the transport of water. These deviations are explained by assuming the existence of a selective facilitated mechanism of water transport. Other possible mechanisms are also discussed.