Alkylation of isobutane with n-butenes to form highly branched isoparaffins is an important process in gasoline manufacture. This reaction is currently catalyzed using either liquid hydrofluoric acid or sulfuric acid. Although these liquid acids rapidly catalyze the alkylation reaction, there are a number of drawbacks associated with their use. Solid acids have been studied extensively as possible catalysts for alkylation. However, with solid acid catalysts, the catalyst deactivates rapidly with time on stream. It is been hypothesized and shown that when the reaction is carried in supercritical media, the solid acid catalyst deactivation slows down considerably; however, high temperatures are needed to bring the reaction mixture to supercritical conditions, which results in the formation of undesired products. In this study, the effects of using different operation conditions for alkylation on a USY zeolite were investigated. It was observed that the deactivation rate was slower in supercritical-phase alkylation. Carbon dioxide was used as a diluent to lower the critical temperature of the reaction mixture in the ratio 27/9/1 for carbon dioxide/isobutene/1-butene. The use of diluent favored both alkylate selectivity and coke precursor removal.