A polymer composite membrane incorporated with a hygroscopic material for high-temperature polymer electrolyte membrane fuel cell (PEMFC) was prepared by hot-pressing the mixture of melt fabricable perfluorosulfonylfluroide copolymer resin (Nation resin) with fine mordenite powder. The composite membranes with various modernite contents were fabricated. The physico-chemical properties of composite membranes, such as tensile strength and proton conductivity were investigated. The proton conductivity in composite membranes increased with increasing mordenite content at high temperature above 90 degreesC due to water in the mordenite. In addition, the results of current-voltage relationship for single cells at various operation temperatures showed that the composite membrane, containing 10wt.% mordenite has better performance than the others. It suggests that the improvements of fuel cell performance are not due simply to increased water content in the composite membrane, but that there is an interaction between proton mobility and structure. (C) 2004 Elsevier Ltd. All rights reserved