Poly(2,5-benzimidazole) (ABPBI)—a promising high-temperature polymer electrolyte membrane—is characterized over a wide range of temperature (−50 to 220 °C) using broadband dielectric spectroscopy (BDS) to understand the various relaxation processes. The undoped ABPBI membrane shows two major secondary relaxations and a primary α relaxation. The effect of phosphoric acid (PA) and phosphotungstic acid grafted zirconium dioxide (PWA/ZrO2) nanoparticles on the chain relaxation and the proton conductivity is investigated. The phosphoric acid alters the relaxation trends, increases the number of free ions in the polymer matrix, and therefore the conductivity. The shift in the peak frequencies of different chain relaxation processes in the presence of PA and PWA/ZrO2 is attributed to the increase in free volume and the consequent easy motion of the polymer chains. The Fourier transform infra-red (FTIR) spectroscopy of ABPBI and the acid-doped composites show all the relevant peaks corresponding to CC, CN stretching, and phosphoric acid/phosphates, confirming the formation of ABPBI and doping with PA. The proton conductivity of the membranes is estimated from electrochemical impedance spectroscopy (EIS). To establish the effect of change in crystallinity on relaxations and proton conductivity, the undoped and PA-doped membranes are characterized using thermogravimetric analysis and in situ XRD at high temperatures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44867. © 2017 Wiley Periodicals, Inc.