We describe zwitterion, 3-(1-butyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (Bimps), mixtures with 1,1,1- trifluoro-N- trifluoromethylsulfonyl)methanesulfoneamide (HN(Tf)2) as new proton transport electrolytes. We report proton transport mechanisms in the mixtures based on results from several methods including thermal analyses, the complex-impedance method, and the pulsed field gradient spin echo NMR (pfg-NMR) method. The glass transition temperature (Tg) of the mixtures decreased with increasing HN(Tf)2 concentration up to 50 mol . The Tg remained constant at -55 A?C with further acid doping. The ionic conductivity of HN(Tf)2 mixtures increased with the HN(Tf)2 content up to 50 mol . Beyond that ratio, the mixtures showed no increase in ionic conductivity (10-4 S cm-1 at room temperature). This tendency agrees well with that of Tg. However, the self-diffusion coefficients obtained from the pfg-NMR method increased with HN(Tf)2 content even above 50 mol for all component ions. At HN(Tf)2 50 mol , the proton diffusion of HN(Tf)2 was the fastest in the mixture. These results suggest that Bimps cannot dissociate excess HN(Tf)2, that is, the excess HN(Tf)2 exists as molecular HN(Tf)2 in the mixtures. The zwitterion, Bimps, forms a 1:1 complex with HN(Tf)2 and the proton transport property in this mixture is superior to those of other mixing ratios. Furthermore, CH3SO3H and CF3SO3H were mixed with Bimps for comparison. Both systems showed a similar tendency, which differed from that of the HN(Tf)2 system. The Tg decreased linearly with increasing acid content for every mixing ratio, while the ionic conductivity increased linearly. Proton transport properties in zwitterion/acid mixtures were strongly affected by the acid species added.