dSulfonic poly(styrene-co-divinylbenzene) ion-exchange resins are commonly employed as supported acid catalysts or as selective sorbents in a number of different applications. Using the reaction of acetic acid and methanol to form methyl acetate and water as a model system, the competitive sorption behavior of the corresponding nonreactive binary pairs was studied for macroporous, highly cross-linked resins of different acid loading. It was found that a reduction of the acid loading decreases the resins' selectivity toward the sorption of water and increases its affinity toward less polar species, e.g., methyl acetate. To describe the sorption behavior a phase equilibrium model based on the Flory-Huggins theory of polymer swelling was developed, taking into account the hydration of the acid sites upon the sorption of water. The ability of the model to represent the competitive sorption behavior as a function of the resin's acid loading has been demonstrated by comparison with various sets of experimental equilibrium data. Finally, initial work to characterize the catalytic performance for different degrees of sulfonation is presented, and conclusions toward the application of these resins as stationary phases for reactive chromatography are drawn.