Zirconium tricarboxybutylphosphonate, Zr(O3PC(CH2)(3)(COOH)(3))(2), was synthesized by slowly decomposing zirconium fluorocomplexes in the presence of 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) to prepare inorganic/organic composite membranes based on polybenzimidazole (PBI). Composite membranes composed of zirconium tricarboxybutylphosphonate (Zr(PBTC)) particles embedded in PBI were formed by evaporating the solvent from a suspension of Zr(PBTC) particles in a PBI solution. Composite membranes with 50 wt.% Zr(PBTC) exhibited uniform distribution of the particles throughout the thickness, as ascertained by scanning electron microscopy. The conductivities measured for compacted Zr(PBTC) powder and PBI-based membranes with 50 wt.% Zr(PBTC) were 6.74 x 10(-2) and 3.82 x 10(-3) S cm(-1), respectively, at 200degreesC for P-H2O/Ps = 1. Functionalization of the PBI with phosphoric acid groups obtained by treatment with phosphoric acid and with sulfonic acid groups obtained by post-sulfonation thermal treatment with sulfuric acid improved the proton conductivity of 50 wt.% Zr(PBTC) membranes to 5.24 x 10(-3) and 8.14 x 10(-3) S cm(-1) under the same measurement conditions. Zr(PBTC)/PBI membranes thus have great potential as proton exchange membrane fuel cells (PEMFCs). (C) 2004 Published by Elsevier B.V.