In this paper, results on the hydrogenation of a-pinene, a liquid reactant, in supercritical (SC) CO2 are presented. Visual observation of phase equilibria under the conditions of the reaction experiments was also performed. Much faster reaction rates were obtained when a two-phase system was present in the reactor than when a homogeneous, one-phase situation occurred. This phenomenon is attributed to the fact that SC CO2 can dissolve extensively in the liquid reactant, leading to the formation of an "expanded liquid", which can dissolve large quantities of hydrogen. This seems to be the first time that this type of behavior is reported in the literature.