The solubilities of isobutylene in the aqueous solutions of p-toluenesulfonic acid have been experimentally determined over wide ranges of acid concentrations, and a method for analyzing the apparent solubilities in terms of the activity coefficients in the pseudocomponents of water-acid mixtures is presented. Near 33 ℃ and 1 atm, the solubilities steadily increased according to the increase of the concentration of acid, pressure, and temperature, while beyond 40 wt%, significant amounts of isobutylene solubilized, in fact, by the acid-catalyzed hydration mechanism forming an intermediate complex. In the equilibrium calculations. van Laar 1-parameter model and Redlich-Kister 3-parameter model, were incorporated with the simplified chemical theory for the pseudobinary systems of isobutylene and pseudocomponent, and the solubility increase was attributed to the chemical complexation.