Composite membranes based on organosiloxane network (OSPN)/sulfonated poly(ether ether ketone) (SPEEK) interpenetrating polymer network (IPN) structures with sulfonated mesoporous benzene-silica (SMBS) proton conductors embedded are fabricated. The flexibility and toughness properties of OSPN are expected to compensate for the brittleness of the sPEEK membranes. The 2D-hexagonal cylindrical mesopore structures of SMBS maintain the water content at a high level to enhance the conductivity, even at low relative humidity. Compared to the pristine sPEEK membranes, the ternary composite membranes can endure about 10 times more elongation before breaking. Both OSPN and SMBS components enhance the proton conductivity of sPEEK membranes in a hydrated state, while maintaining the water uptake at below 55% even at temperatures as high as 100 degrees C. The SAXS patterns of the composite membranes explain the water-related membrane properties of composite membranes. The maximum power densities of Nafion membrane-based MEAs are 178.4 mA cm(-2), 132.2 mA cm(-2), and 90.9 mA cm(-2), but those of composite membrane-based ones are 159.1 mA cm(-2), 134.2 mA cm(-2), and 110.8 mA cm(-2) at 95%, 70%, and 45% relative humidity, respectively. (c) 2013 Elsevier B.V. All rights reserved.