Mass-transport studies of phosphoric acid ͑PA͒-doped meta-polybenzimidazole ͑PBI͒ fuel cell membranes are described. In this study, the fundamental differences in transport properties between m-PBI/PA membranes prepared by conventional imbibing procedures and the polyphosphoric acid ͑PPA͒ process are explored. The membranes were characterized by proton conductivity and multinuclear ͑ 1 H and 31 P͒ magnetic resonance measurements. Both short-range and long-range dynamical processes were investigated by spin-lattice and spin-spin relaxation time measurements and by pulsed field gradient diffusion, respectively. Comparative data for pure PA and PPA are included. The high proton conductivity ͑0.13 S/cm at 160°C͒ of the PPA-processed membranes is correlated with rapid proton self-diffusion ͑3 ϫ 10 −6 cm 2 /s at 180°C͒. The 31 P results reveal the presence of both PA and the dimeric pyrophosphoric acid and indicate strong interaction between the phosphate groups and the m-PBI matrix, with negligible anionic transport for both kinds of membranes. The higher concentration of PA in the PPA-processed membranes and differences in membrane morphology may provide an additional proton-transport mechanism involving rapid exchange between the PA and pyrophosphoric acid species.