Nafion/zirconium phosphate (ZP) composite membranes of different structures containing in situ precipitated ZP, crystalline layer-structured alpha-zirconium hydrogen phosphate alpha-Zr(HPO4)(2)(H2O)-H-. (alpha-ZPL), and three-dimensional-network zirconium hydrogen phosphate H3OZr2 (PO4)(3) (ZPTD) were synthesized. The ZP materials used have different structure and surface properties (specific surface area, zeta potential, and particle morphology). The composite membranes were studied in an H-2/O-2 proton exchange membrane fuel cell (PEMFC) over a range of relative humidity (RH) from 13 to 50% at temperatures of 80 and 120 degrees C. At the operating temperature of 120 degrees C, all studied composite membranes demonstrated significant improvement in performance under dehydrating conditions compared to unmodified Nafion membranes. Among the tested membranes, the Nafion/alpha-ZPL composite membrane demonstrated the highest performance at 120 degrees C at all RH values, especially at 13% RH. At 26 and 50% RH, the performance of Nafion/alpha-ZPL and Nafion/19% ZP in situ membranes were close, and the performance of the Nafion/ZPTD membrane was the lowest. The effects of filled polymer pore constriction, scaffold formation, membrane structure, and surface properties of the ZP particles on the membrane performance in PEMFCs are discussed in the paper. (c) 2007 The Electrochemical Society. All rights reserved.