The selective hydrogenation of cinnamaldehyde (CAL) was investigated using silica supported platinum catalysts in supercritical carbon dioxide. Selectivity to cinnamyl alcohol (COL) is enhanced as Pt-0/Pt2+ ratio increases; namely, zero-valent metallic surface is beneficial to the formation of COL compared with less reduced surface. The influence of Pt-0/pt(2+) ratio is more significant on the selectivity than on the total conversion. For the catalyst with small Pt-0/Pt2+ value, the selectivity also depends on the degree of platinum dispersion. The selectivity to COL is higher for higher degree of platinum dispersion. The CO2 pressure did not affect the conversion and selectivity so much. (C) 2002 Elsevier Science B.V. All rights reserved.