Composite membranes based on sulfonated polyether ether ketone (SPEEK) and hydrated tin oxide (SnO2·nH2O) were prepared and characterized. 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 samples were investigated by simultaneous thermogravimetric and differential thermal analyses (TG-DTA), X-ray diffraction and electrochemical impedance spectroscopy (EIS). Proton conductivity measurements were carried out on the composite systems as a function of temperature and relative humidity in the 20-100% RH range. The polymer electrolyte membranes doped with SnO2·n(H2O) showed an increase in electrical performance that can be attributed to the formation of a coordinative interaction between the water molecule of the metal oxide and -SO3H groups of SPEEK. Conductivity values of the composite SPEEK/SnO2·n(H2O) demonstrated that it possesses good proton transport characteristics and it is then suitable for application as electrolyte in Polymer Electrolyte Fuel Cells. copyright The Electrochemical Society.