a b s t r a c tSynthesis and properties of proton-conducting composites of SPEEK with organically functionalized TiO 2 are described. Composites with hydrophilic titania particles present an inhomogeneous microstructure with agglomeration of TiO 2 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.