The study presented here is a fundamental investigation into the molecular origins of the thermal transitions and dynamic mechanical relaxations of Nafion membranes as studied by DSC, DMA, variable temperature small-angle X-ray scattering (SAXS), and solid-state F-19 NMR spectroscopy. While several studies in the literature have attempted to explain the molecular origins of these thermal transitions and mechanical relaxations, the assignments were based primarily on limited DMA results and have at times been contradictory. In DSC traces of Nafion, the low- and high-temperature endotherms are shown to be dependent on thermal history and are now attributed to melting of relatively small and large crystallites, respectively. DSC analysis of Nafion yields information only on the crystalline nature of this ionomer, and neither of the transitions can be assigned to glass transitions. The intensity of the small-angle ionomer peak at ca. q = 2 nm(-1) was monitored as a function of temperature for each alkylammonium neutralized sample. Changes in intensity of the ionomer peak as a function of temperature were shown to correlate well with the a and P relaxations observed in DMA. Variable temperature solid-state F-19 NMR techniques were used to investigate the dynamics of the Nafion chains. Spin-diffusion experiments revealed a profound increase in mobility at the onset of the a relaxation. Sideband analysis indicated that the side chain is more mobile than the main chain and that the mobility is greatly affected by the size of the counterion. Molecular level information from this analysis in correlation with SAXS and DMA data supports the assignment of the beta relaxation to the genuine T-g of Nafion and the alpha relaxation to the onset of long-range mobility of chains/side chains via a thermally activated destabilization of the electrostatic network.