Nafion and derived perfluorinated polymers seem to be the most reliable materials in PEMFCs, despite a number of disadvantages, such as low operation conditions (T < 100 degrees C) and high cost. In recent years, new classes of proton conducting membranes have been investigated. PBI membranes showed some advantages: high working temperatures; good chemical resistance; promising durability. Proton carriers are supplied by the presence of phosphoric acid in the membranes. Despite the good conductivity (about 0.1 S/cm at 80 degrees C and RH = 80%) one of the main drawbacks is the leaching of the phosphoric acid under working conditions. In this work two different silica-based fillers, derivatised with phosphoric acid and imidazole-containing units, respectively, were prepared via sol-gel route and characterised from a thermal analyses (TGA-DTA), structural (XRD, IR and solid state NMR) and microstructural (BET) point of view. Impedance spectroscopy scans were carried out on PBI-based membranes filled with derivatised silica up to a content of 30 wt% in order to evaluate the role of the filler in the free phosphoric acid retention capability. The best scores in terms of permanent proton conductivity and acid doping level after leaching were obtained in case of imidazole-functionalised silica, which improves the basicity of the doped polymer system. (C) Koninklijke Brill NV, Leiden, 2010