The preparation and solubility of water-methanol mixtures in sulfonated (1,3-dihydro-3-oxoisobenzofuran-1,1-diyl-1,4-phenyleneoxy-1,4-phenylenec arbonyl- 1,4-phenyleneoxy-1,4-phenylene) (C-PEK) was systematically studied. The sulfonation was carried out by reacting C-PEK with 98% sulfuric acid at 60 degrees C. This reaction obeys a first-order reaction mechanism up to 4 h. The sulfonic acid group was w-iiformly dispersed in a sulfonated C-PEK (SCPEK) membrane. The introduction of sulfonic acid groups in C-PEK could increase polymer segmental gaps because of the size of a bulky sulfonic acid group or decrease polymer segmental gaps as a result of polar interactions. The former effect is dominant for SC-PEK with low sulfonic acid content, whereas the latter occurs for SC-PEK with high sulfonic acid content. The uptake of pure water or ao aqueous methanol solution (e.g., 3.5, and 10 wt % methanol concentration) in the sulfonated C-PEK membranes increases with increasing sulfonic acid group content. However, the sulfonated C-PEK with high sulfonic acid content selectively sorbs water rather than methanol in the membrane. This suggests that the sulfonated C-PEK membrane could prevent crossover of methanol through the membrane and has potential as a polymer electrolyte for the direct methanol fuel cell. (c) 2006 Wiley Periodicals, Inc.