H-2/air fuel cell polymer electrolyte membranes, consisting of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI) doped with phosphoric acid, have been studied by H-1 and P-31 magic-angle spinning (MAS) NMR to investigate the polymer-acid interaction and the unresolved question of the nature of the proton conductivity. The H-1 and P-31 resonances have been fully assigned, giving a complete picture of the species present and allowing a study of the local motion of these species by nuclear relaxation. These results have been compared to conductivity measurements, enabling conclusions to be drawn with regard to the molecular motion causing proton conductivity.