High molecular weight, highly PA-doped s-PBI membranes have been developed with robust mechanical properties and excellent proton conductivities (>0.1 S cm⁻¹) at elevated temperatures (>100 °C). These membranes show high PA loadings of 30−35 mol PA/PBI, and average tensile stress and strain of 0.804 MPa and 69.64%, respectively. Proton conductivities were dependent on the acid doping level and measured between 0.1 and 0.25 S cm⁻¹ at 180 °C, a pronounced increase over most phosphoric acid-doped sulfonated PBI membranes to date. Preliminary fuel cell testing with hydrogen fuel and air or oxygen oxidants was performed at temperatures greater than 100 °C without external feed gas humidification and show excellent performance (0.62−0.68 V at 0.2 A cm⁻² and 160 °C, hydrogen/air; 0.69−0.76 V at 0.2 A cm⁻² and 160 °C, hydrogen/oxygen). Initial performance stability studies were conducted for ∼3000 h and indicate great promise as high temperature membranes, with a degradation rate of 30 μV h⁻¹.