Proton mobilities of S-PEEK membranes with variable degrees of sulfonation (DS), and of S-PEEK membranes cast from different solvents were studied using high-resolution solid-state H-1 MAS NMR. In the hydrated S-PEEKs, a single resonance for water-associated sulfonic acid protons was observed for S-PEEK with DS = 58 and 95%, which indicates that the acid protons in either of these S-PEEK samples have similar environments, suggesting a similar distribution of water at each sulfonic acid site. However, there are multiple resonances for water-associated sulfonic acid protons in S-PEEK with DS = 72 and 86%. This suggests distinct acid proton environments resulting from an uneven distribution of water to sulfonic acid protons in these S-PEEK membranes, explaining the fact that these two fully hydrated S-PEEKs show similar conductivities although they have large differences in water content and DS. For S-PEEK membranes cast from different solvents, stronger polymer chain interactions were observed in S-PEEK from N,N-dimethylformamide (DMF) than in those from N,N-dimethylacetamide (DMAc). This could explain why the former exhibits lower proton conductivity although it has the same water content and DS as the latter. S-PEEKs cast from DMAc and dimethylsulfoxide (DMSO) have large differences in water content, but show very similar proton conductivity at high relative humidity. In all cases, solid-state H-1 NMR is shown to be a very sensitive probe of local environments as a function of the casting solvent, degree of sulfonation, and relative humidity, as such spectra provide detailed information about the distribution of acid protons in the membranes. (c) 2008 Elsevier B.V. All rights reserved.