The performance of a poly(2.5-benzimidazole) (ABPBI) membrane based high temperature PEM fuel cell in presence of carbon monoxide, at various temperatures is reported here. The ABPBI was synthesized by polymerization of 3,4-diaminobenzoic acid in a polymerization medium containing methanesulfonic acid (CH3SO3H) and phosphorous pentoxide (P2O5). The ABPBI membranes were characterized by fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). A maximum conductivity of 0.026 S cm(-1) at 180 degrees C was obtained for the membrane doped with 1.2 molecules of phosphoric acid (H3PO4) per polymer repeat unit. Fuel cell performance was evaluated using dry hydrogen/oxygen gases and was comparable with that reported in the literature. Performance of a single cell at different temperatures was studied with 0.48 and 1.0 vol.% of CO in the hydrogen fuel. The studies lead to the conclusion that CO poisoning is not a serious problem above 170 degrees C. Performance of the fuel cell operating at 210 degrees C is not at all affected by 1.0 vol.% of CO in the hydrogen feed. (c) 2005 Elsevier B.V. All rights reserved.