The use of 12-tungstophosphoric acid (HPW) as the ionomer in high temperature, >100 degreesC, proton exchange membrane fuel cells (PEMFCs) was investigated. Since HPW is a crystalline solid, the material is more conveniently studied in membranes fabricated from composites of polymers and HPW. A commercially available high-temperature epoxy was chosen as the polymer for our initial studies. The ionomer was added, either by mixing the HPW with the uncured epoxy or by soaking a cured epoxy membrane in an aqueous solution of HPW. Sulfonated and unsulfonated epoxy membranes, with and without HPW, were fabricated. The structure of the composite membranes was characterized using attenuated total reflectance infrared spectroscopy, small angle X-ray scattering, scanning electron microscopy, and thermal gravimetric analysis. Fuel cell polarization curves were obtained for the membranes under varying conditions of temperature and humidification. A trend of increasing current density was noted with increasing temperature for the HPW-doped sulfonated epoxy membrane. All of the membranes exhibited sufficient mechanical strength to 165 degreesC. Where the sulfonated epoxy, without HPW, failed at temperatures above 165 degreesC, the sulfonated membranes with HPW functioned in the fuel cell to less than or equal to 200 degreesC. (C) 2004 The Electrochemical Society.