Novel proton conducting composite membranes were synthesized by blending Nafion (R) polymer with sulfonated organosilicon dendrimers (G3-S). Blending G3-S with Nafion resulted in stable, flexible membranes with loadings up to 10% by weight. The high amount of sulfonic acid groups on the dendrimer increased the ion exchange capacity and improved the water retention capability of the composite membranes with less swelling. The proton conductivity of the composite membranes was higher at all measured temperatures under both high and low relative humidity conditions, compared to pure Nafion (R). The proton conductivity of Nafion (R) increased from 0.086 S/cm (at 80 degrees C at 100% relative humidity) for the pure polymer to 0.156 S/cm for the 10 wt% loading of dendrimer. (C) 2011 Elsevier B.V. All rights reserved.