Composite Nafion/zirconiurn phosphate membranes were investigated for high temperature operation of proton exchange membrane fuel cells (PEMFCs). The composite membranes were prepared via impregnation of Nation films (either commercial Nation 115 or recast Nation) with zirconyl chloride and 1 M phosphoric acid at 80 degreesC. An MEA employing a composite membrane prepared starting from commercial Nation 115 gave a H-2/O-2 PEMFC performance of about 1000 mA/cm(2) at 0.45 V at a temperature of 130 degreesC and a pressure of 3 bar; this result compares very favorably with the performance of an MEA based on commercial unmodified Nafion, which gave only 250 mA/cm(2) at 0.45 V when operated under the same conditions of temperature and pressure. Similar experiments performed with recast Nation and recast Nafion/zirconium phosphate composites confirmed an analogous improvement of performance of the composite membranes over the unimpregnated ones. In this case, the composite recast Nafion/zirconium. phosphate gave about 1500 mA/cm(2) at 0.45 V at a temperature of 130 degreesC and a pressure of 3 bar. The composite membranes showed stable behavior during time when maintained at 130 degreesC, while irreversible degradation affected Nation under the same conditions.