Inorganic-organic composite electrolytes were fabricated from partially Cs+-substituted heteropoly acids (Cs-HPAs) and sulfonated poly(ether-ether ketone) (SPEEK) for application in fuel cells. Heteropoly acids, such as phosphotungstic acid (H3PW12O40:WPA), and silicotungstic acid (H4SiW12O40:WSiA), were mechanochemically treated with cesium hydrogen sulfate (CsHSO4) to obtain the form of Cs-HPAs. SPEEK was prepared from PEEK by sulfonation using concentrated sulfuric acid. Water durability and surface structure of HPAs were modified by introducing Cs+ into HPAs. Flexible and hot water stable composite electrolytes were obtained, and their electrochemical properties were markedly improved with the addition of Cs-HPAs into the SPEEK matrix. Maximum power densities of 245 and 247 mW cm(-2) were obtained for 50WPA center dot 50CsHSO(4) and 5OWSiA center dot 50CsHSO(4) in SPEEK (1/5 by weight) composite electrolytes, respectively, from single cell tests at 80 degrees C and 80 RH%. These results suggest that a three-dimensional proton-conductive path was formed among homogeneously distributed Cs-HPAs particles in the SPEEK matrix. The Cs-HPAs incorporated into the SPEEK matrix increased the number of protonate sites in the electrolyte. These observations imply that the mechanochemically synthesized Cs-HPAs, which consist of hydrogen bondings between Cs-HPAs and -HSO4-, dissociated from CsHSO4, are promising materials as inorganic fillers in inorganic-organic composite. (C) 2010 Elsevier B.V. All rights reserved.