Nafion/10% (mass % is used in this paper) TiO2 composite membranes were studied in a H-2/O-2 proton exchange membrane (PEM) fuel cell over a range of relative humidity (RH) from 26 to 50% at temperatures of 80 and 120 degrees C. According to the scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) structural analysis, the composite Nafion/TiO2 membranes had a two-layer structure, one layer enriched with TiO2 particles, and the other dominated by the Nafion polymer. Although the TiO2 particles were mainly concentrated on one side of the composite membrane, sufficient hydration was apparently achieved for the whole membrane. Two TiO2 (rutile) powders used for the preparation of the composite membranes differed in specific surface area (SSA), surface zeta potential, and particle morphology. A TiO2 powder with five times higher SSA, 22 mV higher zeta potential (at the low-pH limit), and distinctly different individual particle and particle aggregate morphologies resulted in a four-times increase of current density (at 0.6 V) when the composite membranes were made and tested in PEM fuel cell at temperature of 120 degrees C and relative humidity of 26%. We speculate that a greater number of protonated sites per unit mass of powder in the membrane and a higher density of the protonated sites contribute to the enhanced fuel cell performance. (c) 2005 The Electrochemical Society.