Sol-gel derived Nafion(R)/silica hybrid membranes were investigated as a potential polymer electrolyte for fuel cell applications. Membrane proton conductivity and water content were measured as a function of temperature, water vapor activity, and silica content. The hybrid membranes have a higher water content at 25 and 120 degreesC, but not at 150 and 170 degreesC. Despite the higher water content, the proton conductivities in the hybrid membranes are lower than, or equal to, that in unmodified Nation membranes under all conditions investigated. The proton conductivity of the hybrid membrane decreases with increasing silica content under all conditions.