Microporous poly(tetrafluoroethylene) (PTFE) and Celgard(R) films were used as support material for Nafion(R)-based composite membranes. The microporous supports were impregnated by soaking in a 18 wt% Nafion(R)/ethanol solution. Although there was considerable Nafion(R) uptake by Celgard (43-45 wt%), the conductivity at 65 degrees C of 0.021 S/cm (Celgard(R) 2400) or 0.027 S/cm (Celgard(R) 2500) is relatively low in comparison to that of 0.114 S/cm for Nafion(R) 112. In contrast, a larger uptake (similar to 70 wt%) and conductivity (0.101 S/cm) were measured for the microporous PTFE support. Because the Nafion(R)/PTFE composites are only 31 mu m thick, their areal resistance is lower than that of Nafion(R) 112 membranes : 0.031 vs 0.036 Omega cm(2). Nafion(R) is uniformly distributed across the PTFE support, as evidenced by EDAX visualization of Ag+-exchanged membranes, but does not completely plug the micropores, as suggested by the relatively low bubble-point and the relatively high N-2 and O-2 permeabilities of the composites.