A novel proton-conducting polymer, sulfopropylated poly(benzimidazole) (PBI-PS), was synthesized by the ring-opening reaction of 1,3-propanesultone onto the reactive N-H groups of PBI. The equilibrium water uptake of PBI-PS films increased with increases in the relative humidity and sulfoalkylation level, and the maximum value of water uptake reached 11.3 H2O pel sulfonic acid group (H2O/SO3H) at 90% R.H. and room temperature. The hydrous PBI-PS films showed a high proton conductivity of the order of 10(-3) S cm(-1) in the temperature range from 20 degreesC to 140 degreesC, which is superior to that of conventional proton-conducting polymers.