Composite Nafion-based membranes were prepared and characterized, using an organosilane derivative (sulfonated diphenylsilanediol, SDPSD) as a filler. The physico-chemical and electrochemical properties of the composite membranes were investigated using differential scanning calorimetry (DSC), field emission scanning electron microscopy (FE-SEM), water uptake (W.U.), and electrochemical impedance spectroscopy (EIS). Both conductivity values and thermal features of the composite demonstrated that the presence of SDPSD as a filler significantly modified the properties of the Nation matrix, allowing to increase its performance in terms of proton conductivity as well as thermal stability. In particular, the interaction between Nafion and SDPSD was found to improve the mechanical stiffness of the Nafion matrix, leading to an increase in the temperature range at which the Nafion membrane maintains proper hydration requirements and hence satisfactory proton conductivity values, allowing its use as an electrolyte in Polymer Electrolyte Fuel Cells operating at intermediate temperatures. (c) 2007 Elsevier B.V. All rights reserved.