A modified silane bearing a sulfonic acid function (sulfonated diphenylsilanediole, SDPSD) was prepared and characterized. The resulting ormosil was mixed with sulfonated polyetheretherketone (SPEEK) with high degree of sulfonation (0.9) leading to homogeneous composite membranes. The formation of the composite substantially modified the properties of SPEEK in terms of water uptake and solubility. The structural and electrochemical performance of the components and of the composite were investigated with thermogravimetric analysis, field emission scanning electron microscopy, H-1 and Si-29 nuclear magnetic resonance, both in solution and in the solid state, and electrochemical impedance spectroscopy. Both conductivity values, as high as 0.1 S cm(-1), and H-1 diffusion coefficients of the composite SPEEK/SDPSD demonstrated that it possesses good proton transport characteristics up to 120 S C, and it is then suitable for application as an electrolyte in polymer electrolyte fuel cells operating at intermediate temperature. (C) 2006 The Electrochemical Society.