Polybenzimidazole (PBI) has become a popular polymer of choice for the preparation of membranes for potential use in high-temperature proton exchange membrane polymer fuel cells. Phosphoric acid-doped composite membranes of poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (PBI4N) impregnated with hafnium oxide nanofiller with varying content levels (0-18 wt %) have been prepared. The structureproperty relationships of both the undoped and acid-doped composite membranes are studied using thermogravimetric analysis, modulated differential scanning calorimetry, dynamic mechanical analysis, wide-angle X-ray scattering, infrared spectroscopy, and broadband electrical spectroscopy. Results indicate that the presence of nanofiller improves the thermal and mechanical properties of the undoped membranes and facilitates a greater level of acid uptake. The degree of acid dissociation within the acid-doped membranes is found to increase with increasing nanofiller content. This results in a conductivity, at 215 degrees C and a nanofiller level x = 0.04, of 9.0 x 10(-2) S cm(-1) for [PBI4N(HfO2)(x)](H3PO4)(y). This renders nanocomposite membranes of this type as good candidates for use in high temperature proton exchange membrane fuel cells (HT-PEMFCs).