The polyethylene membranes are hydrophilized under the action of chlorosulfonic acid. Such a hydrophilized membrane shows much better water permeability in comparison with the unmodified one. The ion-exchange capacity of the sulfonated membrane is taken as a measure for the hydrophilicity. The influences of the concentration of the sulfonating agent, temperature, and reaction time, on the ion-exchange capacity and water permeability, are investigated. It is shown that the increase in the degree of sulfonation does not always lead to an increase in the water permeability of the membrane. A lower concentration of the sulfonating agent and a lower temperature are to be chosen, so that only the surface and the inner walls of the pores are sulfonated. Sulfonation in the bulk leads to the deterioration of mechanical properties of the membrane without having any contribution to the increase in the water permeability of the membrane.