Crosslinked metal oxide containing nanocomposite membranes, in which the filler also acts as crosslinker, were prepared by blending polybenzimidazole (PBI-OO) and phenylsulfonated TiO2 particles (s-TiO2). Thermal curing changes the ionically crosslinked system into a covalently crosslinked system. The synthesized s-TiO2 nanoparticles were analyzed by thermal gravimetric analysis and scanning electron microscopy. The covalently crosslinked nanocomposite membranes (c-sTiO(2)-PBI-OO) were doped with phosphoric acid (PA) for high temperature proton exchange membrane fuel cell (HT-PEMFC) application. The membrane properties, such as PA uptake, dimensional change, gel content, proton conductivity, mechanical property, and single cell performance were evaluated and compared with the properties of acid-doped c-PBI-OO. PA doped 6-c-sTiO(2)-PBI-OO (6 wt% sTiO(2)) showed the highest uptake of 392 wt%, and a proton conductivity at 160 degrees C of 98 mS cm(-1). In the fuel cell, a peak power density of 356mWcm(-2) was obtained, which is 76% higher than that of a c-PBI-OO based system (202mWcm(-2)). To evaluate the stability of the membrane performance over time, the best performing membrane was tested for over 700 h.