The effect of hydrogen partial pressure on the hydrogenation of limonene in high-pressure carbon dioxide, catalysed by carbon-supported palladium, was studied. Experiments were performed at various hydrogen pressures in the range from 2.5 up to 4.4 MPa at a fixed total pressure of 12.5 MPa, where the reaction mixture remains biphasic-liquid + gas. In these conditions, the liquid phase volume is expanded in relation to the initial volume of limonene in a fashion that is strongly dependent on the hydrogen to carbon dioxide pressure ratios. Surprisingly, the main factor in the kinetics of hydrogenation turns out to be the concentration (mol dm(-3)) of limonene, which decreases due to the volumetric expansion of the liquid phase when the CO2 pressure increases. (C) 2007 Elsevier B.V. All rights reserved.