Sulfonated poly(vinyl fluoride) (PVF-SA) has been made by chemical sulfonation or radiation-induced sulfonation of commercial poly(vinyl fluoride) (PVF) films. The effects of the irradiation treatment and sulfonation on sulfonic acid distribution, crystallinity, state of water, and molecular organization have been examined. The results indicate that proton irradiation and subsequent sulfonation produce a structure that is different from the ones produced by the sulfonation of nonirradiated or electron beam (EB)-irradiated samples. The water uptake is higher in proton-irradiated samples than in the other samples. In addition, the portion of nonfreezing water is highest in proton-irradiated samples. Infrared spectra of the sulfonated samples indicate that a large part of the freezing bound water is associated with the hydrophobic polymer backbone. However, this portion was smaller in the proton-irradiated sample than in the EB-irradiated sample. The proton-irradiated samples had a small-angle X-ray diffraction maximum with a corresponding Bragg spacing of 70 Å, which was taken as evidence for the formation of ion-water cluster domains in the protonirradiated samples. The ion conductivity was slightly lower in nonirradiated and in EB-irradiated membranes than in the proton-irradiated sulfonated samples in which the highest values were 10 -20 mS/cm.