Ammonium-exchanged membranes were prepared and utilized for the dehydration of ethanol/water mixtures by pervaporation. This paper presents an original investigation on the influence of the ammonium content on the hydrophilic and swelling properties of ion-exchange membranes. The identification of ammonium was carried out by infrared ray (IR) spectroscopy. It was found that the content of ammonium did not significantly change the membrane surface morphology but considerably decreased the water contact angle for the membranes. The optimum ammonium substitution in the membrane resulted in a high pervaporation performance for the dehydration of ethanol solution. More than 99% of permeate water was obtained in the pervaporation process. Based on the sorption and diffusion mechanism discussion, the high selectivity of water to ethanol was mainly contributed by the diffusion rate difference of permeants in the membrane rather than the sorption. The hydrophilicity improvement and decrease in swelling of modified membranes contributed to the high selectivity of ion-exchanged membranes for pervaporation. An excellent dehydration performance of ion-exchanged membranes was observed. All the PSI values of the ion-exchanged membrane were higher than 105, which could be attributed to the suitable conditions applied for preparing ion-exchanged polysulfone membranes.