The thermodynamics of acid group hydration was studied for different membranes (including Nafion (R) and sulfonated poly(arylene ether sulfone) (BPSH)) and model systems (including an organic/inorganic composite (I/O) and a multiblock polymer (MB-150)). Experiments were carried out with the membranes exposed to different activities of water corresponding to different level, of membrane hydration. Isopiestic sorption shows a significantly greater uptake in water for the multiblock polymer as compared to the others. Differential scanning calorimetry (DSC) was used to study thermal properties to elucidate the state of water in the membranes. A comparison is made of various modes of collecting data using DSC. Based on these data, we discuss the overall understanding of water interactions with these membranes as well as the limitations of thermodynamic data in describing microenvironments within the membrane. (C) 2007 Elsevier B.V. All rights reserved.