The alkylation of toluene with isopropyl alcohol under the operations in gaseous nitrogen, supercritical CO2, and liquid and supercritical toluene over the parent and modified HZSM-5 pellets was carried out in a packed bed. The chemical liquid deposition technique with the deposition agent SiCl4 was used to modify the HZSM-5 pellets. The experimental results at all of the operations indicated that the modification was essential to enhance the p-cymene selectivity and yield. Because of the limitation of the solubility of water in supercritical CO2, the operating pressure far away from the CO2 critical pressure was required to effectively remove water from the catalyst pellets. The p-cymene selectivity and yield were found to be of 94% and 84%, respectively, at a temperature of 543 K, a pressure of 17.23 MPa, a weight hourly space velocity of 6.5 (g of toluene)/(g of catalyst)/h, and a molar ratio of toluene to isopropyl alcohol of 11 using the modified HZSM-5 pellets. The p-cymene selectivity at this operation condition was slightly higher than that under atmospheric pressure operation (92%), but the p-cymene yield was much enhanced compared to that under atmospheric pressure operation (51%). For the operations in liquid and supercritical toluene, a steady-state operation was not achieved, resulting from an inefficient removal of water from the catalyst pellets. Besides, more side products were generated when the operating temperatures exceeded the toluene critical temperature.