The nature of water in proton exchange membranes, obtained by radiation grafting of styrene onto poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films and subsequent sulfonation, was investigated by differential scanning calorimetry. It was found that the membranes contained three different types of water, which were categorized as freezing-free, freezing-bound, and nonfreezing water. The degree of grafting had a considerable influence on the relative ratios of these three types of water molecules. The nonfreezing water per ion-exchange site ratio was found to be independent of the degree of grafting. However, freezing-free and freezing bound water per ionic site ratios increased with increasing graft levels. A correlation between the water uptake and the structural changes occurring in the membrane as a function of the degree of grafting was established.