The transalkylation of diethylbenzene with benzene to produce ethylbenzene (EB) was studied in order to establish the effect of the reaction conditions, pressure, temperature and contact time, as well as the influence of media reaction phase, supercritical or subcritical on catalysts performance. Both CO2 and free-solvent systems, using an acidic commercial Beta zeolite as catalyst, have been explored. The results showed that both temperature and contact time (W/F) had a positive effect on the ethylbenzene yield, while pressure had a negative one that was more significant at low temperatures. Supercritical conditions only improved the catalyst performance in the free-solvent system. Catalysts tested in SC phase with CO2 showed deactivation while the catalyst tested in SC phase without CO2 maintained the activity. These results have been explained in terms of the different densities and diethylbenzene diffusion coefficients of the reaction mixture that determine the solubility of the coke precursors compounds, and thus catalyst deactivation. (C) 2008 Elsevier B.V. All rights reserved.