Sulfonated styrene - butadiene-styrene (SSBS) triblock copolymer was cross-linked through condensation of sulfonic acid groups with bis(4-aminophenyl)sulfone to form sulfonamide linkages using a coupling agent (carbonyldiimidazole). Intact and clear light brown membranes were obtained when cast from solution, and cross-linking was established by measuring percent water uptake, determined gravimetrically and by near-IR (NIR) spectroscopy, and percent sulfonation by titration. The cross-linked SSBS membranes were found to absorb less water, showed higher resistance to thermal degradation and, when not highly cross-linked, showed better membrane integrity when immersed in water for extended periods. The percent sulfonation of the cross-linked SSBS membranes was found to be 2-10% lower than the non-cross-linked membranes.