Nafion(R)-MO2 (M = Zr, Si, Ti) nanocomposite membranes were synthesized with the goal of increasing the proton conductivity and water retention at higher temperatures and lower relative humidities (120 degrees C, 40% RHs) as well as to improve the thermo-mechanical properties. The sol-gel approach was utilized to incorporate inorganic oxide nanoparticles within the pores of Nafion(R) membrane. The membranes synthesized by this approach were completely transparent and homogeneous as compared to membranes prepared by alternate casting methods which are cloudy due to the larger particle size. At 90 degrees C and 120 degrees C, all Nafion(R)-MO2 sol-gel composites exhibited higher water sorption than Nafion(R) membrane. However, at 90 degrees C and 120 degrees C, the conductivity was enhanced in only Nafion(R)-ZrO2 sol-gel composite with a 10% enhancement at 40% RH over Nafion(R). This can be attributed to the increase in acidity of zirconia based sol-gel membranes shown by a decrease in equivalent weight in comparison to other nanocomposites based on Ti and Si. In addition, the TGA and DMA analyses showed improvement in degradation and glass transition temperature for nanocomposite membranes over Nafion(R). (C) 2005 Elsevier Ltd. All rights reserved.