Development of alternate materials to Nafion, based on ionically conducting polymers and their blends is important for the wider applications of proton exchange membrane fuel cells. In this work, blends of sulfonated poly(ether ether ketone) (SPEEK) with poly(ether sulfone) (PES) are investigated. SPEEK with various ion exchange capacity (IEC) was prepared and blended with PES, which is nonionic and hydrophobic in nature. A comparative study of the water uptake, proton conductivity, and thermo-mechanical characteristics of SPEEK and the blend membranes as a function of the IEC is presented. Addition of PES decreases the water uptake and conductivity of SPEEK. Chemical and thermal stability and mechanical properties of the membrane improve with the addition of PES. The effect of water content on the thermo-mechanical properties of membranes was also studied. The morphology of blend membranes was studied using SEM to understand the microstructure and miscibility of the components. On the basis of the results, a plausible microstructure of the blends is presented, and is shown to be useful in understanding the variation of different properties with blending. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013