Sulfonated polyether ether ketone (SPEEK)-based composite membranes doped with hydrated tungsten oxide were prepared and studied for proton exchange membrane applications. Hydrated tungsten oxide (WO(3)(.)2H(2)O) was synthesized via acidic hydrolysis of sodium tungstate and its structure and physicochemical features were investigated by thermogravimetric analysis (TG), X-ray diffraction (XRD), and electrochemical impedance spectroscopy (EIS). SPEEK/WO(3)(.)2H(2)O composite membranes were prepared by mixing proper amounts of SPEEK and hydrated WO3 in dimethylacetamide as casting solvent. The composite membranes were characterized by XRD, TG-DTA, EIS, and water uptake measurements as a function of the oxide content in the membrane. In particular, XRD patterns as well as TG measurements indicated the existence of a coordinative interaction between the water molecules of tungsten oxide and the SPEEK sulfonic acid groups. This interaction lead to the enhancement of the membrane proton conductivity, as well as of their properties, from the point of view of heat resistance and water solubility. In fact, the addition of tungsten oxide resulted in higher proton conductivity, improved heat resistance, and lower water solubility. (c) 2006 The Electrochemical Society.