Mechanical, thermal and electrical properties of polymer blends of SPEEK with 7% SiPPSU are reported. Presence of silicon stabilizes the polymer morphology. The glass transition temperature, determined by dynamic mechanical analysis, depends more on blend formation than on degree of sulfonation. The water uptake coefficients, determined in liquid water and water vapour, are strongly reduced by the presence of the secondary silylated phase, so that blend membranes do not dissolve in water even at 140 degrees C. The electrical conductivity is of the order of pure SPEEK and does not degrade even at high relative humidity. The sum of these properties make these self-assembled nanocomposite membranes most attractive for use in intermediate temperature PEMFC. (C) 2009 Elsevier B.V. All rights reserved.