Sulfonated poly(ether ether ketone)s (SPEEKs) were substituted on a polymer main chain that had previously been prepared by sulfonation of poly(ether ether ketone)s in concentrated sulfuric acid for a specified time. The product was then blended with Nafion (R) to create composite membranes. The blended SPEEK-containing membranes featured flaky domains dispersed in the Nafion (R) matrix. These blends possessed a high thermal decomposition temperature. Additionally, owing to the more crystalline, the blended membranes had a lower water uptake compared to recast Nafion (R), the methanol permeability was reduced to 1.70 x 10(-1) to 9.09 x 10(-7) cm(2) s(-1) for various SPEEK concentrations, and a maximum proton conductivity of similar to 0.050 S cm(-1) was observed at 30 degrees C. The single-cell performances of the Nafion (R)/SPEEK membranes, with various SPEEK concentrations and a certain degree of sulfonation, were 15-25 MWcm(-2) for SPEEK53 and 19-27 MWcm(-2) for SPEEK63, at 80 degrees C. The power density and open circuit voltage were higher than those of Nafion (R) 115 (power density = 22 mWcm(-2)). The blended membranes satisfy the requirements of proton exchange membranes for direct methanol fuel cell (DMFC) applications. (C) 2008 Elsevier B.V. All rights reserved.