A series of silane based nanostructured composite membranes were synthesized from 3-glycidoxy propyl trimethoxy silane (GPTMS) and 2(3, 4 epoxy cyclohexyl) ethyl triethoxy silane (EHTES) with varying proportion of aqueous orthophosphoric acid, nanoclay (Cloisite((R)) 30B) and multi-walled carbon nanotubes (MWNTs) by sol-gel method. Thermal stability of the composite membranes studied by thermogravimetric analysis (TGA) showed that the composite membranes were stable up to 200 degrees C. However, the thermal stability decreased with an increase in GPTMS proportion and phosphoric acid content but it increased with increasing proportion of nanoclay and nanotubes in the membranes. The improved thermal stability of the membranes was due to the formation of C-O-C bonds and Si-O-Si bonds between the two silanes as confirmed from FT-IR studies. The proton conductivity of the membranes, measured at four different temperatures (80 degrees C, 100 degrees C, 120 degrees C and 140 degrees C) and at 30% relative humidity, was found to increase with an increase in weight proportion of EHTES, orthophosphoric acid, Cloisite((R)) 30B and MWNTs in the composites. The increase in proton conductivity with the increase in wt. proportion of nanoclays and nanotubes in the composites membranes was due to the presence of chemisorbed water in the composites as confirmed by differential scanning calorimetry (DSC). Phase morphology studied by scanning electron microscopy (SEM) showed that the membranes were of dense structure and the nanoparticles were distributed uniformly through out the polymer matrix. (c) 2007 Elsevier B.V. All rights reserved.