Acid-doped polybenzimidazole membranes with good electrochemical properties at high temperatures have been under increasing study in recent years for application in high temperature polymer electrolyte membrane fuel cells. The disadvantages associated with these types of membranes include reduced mechanical properties due to doping and acid leaching under continuous usage. The aforementioned drawbacks can be partly overcome by the addition of modified inorganic nanoparticles. This work reports the synthesis of poly(2,5-benzimidazole) (ABPBI) membranes and fabrication of ABPBI-silica nanocomposites by an ex situ technique using methanesulfonic acid as the solvent. The introduction of sulfonated silica nanoparticles into the ABPBI matrix helps to improve the water uptake, proton conductivity, thermal stability and mechanical property of the nanocomposites compared to the virgin ABPBI membrane. The storage modulus of phosphoric acid-doped ABPBI (dABPBI) nanocomposite with 10 wt% modified silica is increased by 2-fold compared to that of virgin dABPBI. The maximum proton conductivity of 38.01 mS cm(-1) is obtained from dABPBI nanocomposite membrane containing 10 wt% modified silica at 140 degrees C and 1% relative humidity in comparison to 15.23 mS cm(-1) for virgin dABPBI membrane under similar conditions. (c) 2012 Elsevier B.V. All rights reserved.