Solution cast films of lightly sulfonated styrene-b-[ethylene-co-butylenel-b-styrene, (sSEBS) block copolymers were annealed for various times at 120 degreesC and thermal transitions are evaluated using dynamic mechanical analysis. Increased annealing time and increase in degree of sulfonation increases T-g for the PS phase while T-g for the EB phase is practically unchanged, and in some cases, there is suggestion of a relaxation due to EB-PS inter-phases. Annealing has a minor effect on the rubbery plateau storage modulus. Thus, annealing primarily alters the PS block phase. EB-PS phase separation appears to be refined with increasing SO3H content. The region of rubber elasticity extends to higher temperatures with increased degree of sulfonation. A high temperature dynamic mechanical transition that is tentatively attributed to disruption of SO3H-rich sub-domains within the PS block domains shifts to higher temperature with annealing. (C) 2004 Elsevier Ltd. All rights reserved.