Results on the kinetics of hydrogenation of α-pinene in high-pressure carbon dioxide, using Pt/C (1%) catalysts, are presented. Experiments were performed at different carbon dioxide pressures, so that the reaction mixture in contact with the solid catalyst would either be biphasic (liquid + gas) or a single supercritical phase. The technique involved the use of a high-pressure view cell, which allows direct visual observation of the number of phases in the reactor. The hydrogenations in biphasic conditions, at lower carbon dioxide pressures, are completed faster than in supercritical conditions. The results indicate that the pinene adsorption to the metal in the catalyst is the rate-controlling step.