The effect of reducing additive particle size on the conductivity of Nafion (R)/phosphotungstic acid composite membranes was evaluated. The additive particle size was reduced by nearly two orders of magnitude from 1 to 2 mu m to 30 nm by appropriately modifying the casting procedure. The membranes prepared were characterized by transmission electron microscopy and atomic force microscopy to determine particle size. Measurements were performed to determine the hydrogen crossover current through and the area-specific resistance of the membranes. The composite membranes had crossover currents on the order of 1-5 mA/cm(2), with the crossover flux decreasing and approaching the value for recast Nafion (R) as the particle size was reduced. A 25 mu m thick composite membrane with large additive particles had an area-specific resistance of 0.22 Omega/cm(2) at 120 degrees C and 35% relative humidity, while the corresponding value for a 25 mu m thick membrane with small additive particles was 0.16 Omega/cm(2). The latter membrane compared favorably with recast Nafion (R), which had an area-specific resistance of 0.19 Omega/cm(2) under these conditions. It is postulated that the reduction of additive particle size results in an enhancement in the surface to volume ratio of the additive, which in turn permits more efficient proton hopping and hence larger proton conductivities in the composite membrane. The performance of membrane electrode assemblies prepared using these composite membranes at 120 degrees C and 35% relative humidity is also presented. (c) 2005 Elsevier B.V. All rights reserved.