Heteropolyacids (HPAs) are nano-size metal-oxygen clusters with anion acids stronger than the conventional solid acids. Insoluble and high surface area solid acids, i.e. Cs2.5H0.5PW12O40 (CsPW) and H3PW12O40/SiO2 (PWS) are dispersed throughout End-group cross-linkable, Sulfonated Fluorinated Bi-Phenol (ESF-BP) copolymer to produce organic-inorganic nanocomposite proton exchange membranes (PEMs). The molecular structure of ESF-BP copolymer is identified by H-1 NMR spectra. The FT-IR of prepared CsPW and PWS inorganic powders show that the primary Keggin structure remains almost unaltered. The PEMs properties including density, water uptake, water diffusivity, swelling, ion exchange capacity (IEC), oxidative stability and proton conductivity are investigated to characterize the influence of added HPAs on the composite PEMs. The intermolecular interaction between HPAs and the sulfonic group of the copolymer affects the physico-chemical characteristics of membranes. The addition of HPAs to the polymer matrix increases oxidative stability, water uptake and conductivity but decreases the IEC, the water diffusivity and the density of membrane. The cells with the nanocomposite membranes have better performance than the cell with the plain polymer membrane. The prepared composite membrane using dispersion of PWS through the ESF-BP has superior characteristics and voltage-current responses in the PEMFC rather than using CsPW. (C) 2011 Elsevier Ltd. All rights reserved.