A new kind of inorganic-organic composite membranes based on sulfonated polyethersulfone Cardo (SPES-C) with embedded phosphotungstic acid (PWA) was prepared. The composite membranes were characterized by using FT-IR, XRD, TGA and SEM techniques. FT-IR and XRD measurements confirmed the presence of PWA particles into the polymer matrix. FT-IR spectra indicated the existence of a specific interaction between PWA particles and SPES-C polymer. TGA results showed that the composite membranes were thermally stable up to approximately about 200 degrees C. SEM micrographs verified. that nano-scale PWA particles were homogeneously distributed into the polymer matrix. By incorporation PWA into SPES-C polymer the proton conductivity of the composite membranes was enhanced. It was 4.5 x 10(-2) S cm(-1) at 90 degrees C and 6.7 x 10(-2) S cm(-1) at 110 degrees C, while the conductivity of Nafion (R) 115 membrane was 4.1 x 10(-2) S cm(-1) and 4.7 x 10(-2) S cm(-1) at 90 degrees C and 110 degrees C, respectively. (c) 2006 Elsevier B.V. All rights reserved.