Results of atomistic simulations of neat, hydrated, and phosphoric acid (PA)-doped poly(2,5-benzimidazole) (ABPBI) are reported in this article, with an emphasis on the effect of water concentration, PA-doping level, and temperature on the hydrogen bond network and vehicular diffusion. Analysis of radial distribution functions indicates that varying water concentrations and PA-doping levels does not have a significant effect on hydrogen bond lengths; however, high temperature causes hydrogen bond lengths to increase reflecting lower bond strength. Calculations of the number of hydrogen bonds indicate those involving PA increase with PA-doping level, decrease with increasing temperature, and increase with decreasing water content. Sorption calculations using Grand Canonical Monte Carlo indicate the number of water molecules absorbed per 100 ABPBI repeat units is 141.5 at 298 K, in agreement with the experimental value. Vehicular diffusion coefficients in PA-doped ABPBI decrease in the order: water > PA > ABPBI. Specifically, the vehicular diffusion of PA increases with PA-doping level, temperature, and water content. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers