Organic solvents expanded with supercritical. carbon dioxide can be excellent media for hydrogenation reactions. However, catalyst poisoning by CO formed via the reverse water-gasshift reaction occurs during many hydrogenations in the presence Of CO2. In this research, the hydrogenation of polystyrene in CO2-expanded decahydronaphthalene was studied in a batch reactor using two hydrogenation catalysts, 5%Pd/BaSO4 and 65%Ni/Al2O3/SiO2. The 5%Pd/BaSO4 catalyst deactivated at 150 degrees C and CO2 pressures of 250-2250 psig (1.8-15.6 MPa). Approximately 50 ppm CO was present in the CO2-rich light phase after about 10 h at 150 degrees C, 750 psig H-2 pressure, and 2250 psig CO2 pressure. A model that incorporates CO poisoning was developed to describe deactivation of the Pd/BaSO4 catalyst. The 65%Ni/Al2O3/SiO2 catalyst was more active for ring hydrogenation than 5%Pd/BaSO4, and very little CO was formed in the presence of CO2. The Ni catalyst deactivated in the presence Of CO2 at 180 degrees C, possibly due to H2O formed in a methanation reaction.