The importance of the blend microstructure and its effect on conductivity and structural integrity of proton exchange membranes (PEM) were investigated. Sulfonated poly(ether ketone ketone) (SPEKK) was selected as the proton-conducting component in a blend with either poly(ether sulfone) (PES) or SPEKK with a different sulfonation level. The second component was added to improve the mechanical stability in the fuel cell environment. Membranes were cast from solution using N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc). Special attention was paid to the ternary solution behavior. Solution cast SPEKK/PES membranes are homogeneous for all studied compositions, 8/2 through 515 (w/w), and sulfonation levels, 1.7-3.5 mequiv./g. Although this polymer pair does not show evidence for intrinsic compatibility, the excellent solvent quality results in a frozen-in structure during solution casting. The morphology of SPEKK/SPEKK blends can be tailor-made by finding the right balance between composition, casting solvent and temperature. Co-continuous morphologies can be devised for an SPEKK blend with sulfonation levels of 1.2 and 2 mequiv./g. Both blends show lower swelling than the parent SPEKK. This results in better stability of PEMs during fuel cell testing. (c) 2005 Elsevier B.V. All rights reserved.