Composite membranes prepared by incorporating SiO2, TiO2, or Zr(HPO4)(2) particles into Nafion exhibit enhanced performance in proton exchange membrane (PEM) fuel cells tested at elevated temperatures compared to filler-free Nafion when operating under lower relative humidity. Water-transport properties of these membranes were investigated by nuclear magnetic resonance (NMR) methods, including pulsed-field-gradient spin-echo diffusion, spin-lattice relaxation, and spectral measurements. At least two distinct water environments are observed in all materials, and the enhanced water uptake of the composites, relative to filler-free Nafion, is attributed to alteration of the pore structure of the membrane. (c) 2007 The Electrochemical Society.