The thermodynamic study of the sulphonation of polystyrene-butadiene rubber (PSBR) with chlorosulphonic acid for fuel cell application was carried out. This was done to understand the properties of the sulphonation process in terms of the change in enthalpy (Delta H), entropy (Delta S), and the Gibbs free energy (Delta G) relative to product stability. The Delta H-0, Delta S-0, and Delta G(0) were found to be 40.708 kJ, 64.22 J K-1, and 22.916 kJ, respectively, indicating that the reaction is not thermodynamically favorable (though non-sponteneous) where products are less favored compared to reactants which thus required energy. However, at elevated temperature (>328 K), a phase change from liquid to solid was encountered which suggests that the optimum temperature to carry out the sulphonation of PSBR in chlorosulphonic acid should be within 328 K. Results obtained also revealed that the sulphonation time influenced the qualities such as, proton conductivity, degree of sulphonation, and methanol permeability of the sulphonated PSBR. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 1766-1771, 2010