Inorganic nanoparticles in nanocomposite membranes significantly affect the characteristics of those membranes, such as proton and methanol transport behavior, membrane durability, and electrochemical single cell result. Therefore, the inorganic nanoparticles should be deliberately chosen to fabricate composite membranes with desirable properties for DMFC. In this study, sulfonated poly(arylene ether sulfone) (SPAES) and hydrophilic fumed silica (SiO2) were used as a polymer matrix and an inorganic nanoparticle, respectively. The SiO2 nanoparticles have various surface areas (150, 200, 300, and 380 m(2) g(-1)) and average particle sizes (7, 12, and 14 nm). The SiO2 nanoparticles are evenly dispersed in the SPAES matrix by aid of a non-ionic surfactant (Pluronics(R) L64). Interestingly, SiO2 particles with a high surface area and small particle size showed the best results: high proton conductivity, long membrane life time under oxidative conditions, good dimensional stability, outstanding single cell performance, and reduced methanol crossover. Moreover, SiO2 content plays an important role in membrane microstructures and membrane properties such as proton conductivity and methanol barrier behavior. An excessive SiO2 content caused a large aggregation of SiO2 particles, leading to the deterioration of mechanical properties in nanocomposite membranes. In the present study, optimal SO2 content for maximizing the fuel cell performance of current nanocomposite membranes was ca. 2 wt.%. (C) 2007 Elsevier B.V. All rights reserved.