Synthesis and properties of proton-conducting composites of SPEEK with organically functionalized TiO2 are described. Composites with hydrophilic titania particles present an inhomogeneous microstructure with agglomeration of TiO2 particles, high strength and low ductility, high water uptake and proton conductivity. Composites with hydrophobic titania particles have a very homogeneous microstructure, very reproducible mechanical properties, lower water uptake and proton conductivity. (C) 2011 Elsevier B.V. All rights reserved.