The physiochemical properties of Nation 115 and a composite Nation 115/zirconium phosphate (similar to25 wt.%) membranes are compared. The composite membrane takes up more water than Nation at the same water activity. However, the proton conductivity of the composite membrane is slightly less than that for Nation 115. Small angle X-ray scattering shows that the hydrophilic phase domains in the composite membrane are spaced further apart than in Nation 115, and the composite membrane shows less restructuring with water uptake. Despite the lower proton conductivity of the composite membranes they display better fuel cell performance than Nafion 115 when the fuel cell is operated at reduced humidity conditions. It is suggested that the composite membrane has a greater rigidity that accounts for its improved fuel cell performance. (C) 2004 Elsevier B.V. All rights reserved.