Polymeric proton electrolytes based on the composite of 12-tungstosilicic acid (12-TSA) and polyethylene oxide (PEO) have been prepared. X-ray diffraction (XRD) and infrared spectroscopy indicate that 12-TSA in the composite is complexed with the polymer to form a new compound. The proton conductivity of the composite polymer electrolyte depends on the concentration of the 12-TSA, the relative humidity and the temperature. The highest conductivity at room temperature has been found to be 6.3 x 10(-2) S cm(-1) for the composite with an [H+]/[EO] mole ratio of 0.025. (C) 2003 Elsevier Science B.V. All rights reserved.