In the first systematic study of the influence of the size of silica particles on ion exchange capacity (IEC) and proton conductivity of Nafion-silica nanocomposite membranes, thin films cast from mixtures of silica particles (5 wt%) with varying diameters ranging from 10 to 400 nm and Nafion in alcohol were examined. IECs decreased exactly as expected with the dilution of Nafion and its sulfonic acid groups with added silica. At 80 degrees C, the proton conductivity was also less with silica particles. However, at higher temperatures (120 degrees C), there was a 58 % improvement in proton conductivity at low relative humidity and a 45 % improvement at higher relative humidity for nanocomposite membranes prepared with silica particles <50 nm in size. The improvement was less significant with larger silica particles in the membranes.