Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion-based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. The composite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long-time creep of similar to 40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3-20 wt% titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of similar to 0.55 MPa in 125-mu m membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 degrees C and 100% relative humidity, the resistivity of Nafion increases by similar to 15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. (c) 2006 Wiley Periodicals, Inc.