In the aqueous solutions of p-toluenesulfonic acid, ethylbenzenesulfonic acid, dodecylbenzenesulfonic acid, and sulfuric acid, the solubilities of isobutylene were determined by the volumetric method at 33℃. Following the pseudobinary complexation model, the activities of isobutylene were determined for all acids, based on the Wilson model incorporated with the chemical theory. In solubilizing isobutylene, the significant increase of the equilibrium constant K, and the chemical conversion to tert-butyl alcohol suggested that the chemical complexation of isobutylene with acids might play a key role in the mechanism of the solubilization. By increasing the chain length of alkylbenzene-sulfonic acids, it is also found that the efficiencies of acids in solubilizing isobutylene were enhanced at lower concen-trations, but significantly reduced at higher concentrations. After a transition concentration where there appeared minima, the efficiencies of acids were greatly enhanced, indicating that the chemical conversion to t-butyl alcohol istriggered by acids. Further, the solubilizing efficiencies of mixed acids consisting of p-toluenseulfonic acid and dodecylbenzenesulfonic acid, linearly decreased depending on the effective chain hength at a fixed molar concentra-tion of acids, and it may be attributed to the formation of complex by acid rather than to the incorporation of isobutyl-ene into the micelles.