Selective hydrogenation of cinnamaldehyde has been studied in scCO(2) medium using Pt-supported mesoporous MCM-48 catalysts prepared by two different techniques: impregnation and in Situ addition employing different Pt precursors. Characterization of those catalysts by several spectroscopic techniques showed that the metal particle size as well as the oxidation state differ considerably depending on the synthetic method. All the catalysts were found to be active and selective to the hydrogenation of cinnamaldehyde in scCO(2) irrespective of the synthetic method. The in situ catalyst was highly selective to the cinnamyl alcohol without any prior reduction, whereas the impregnated catalyst produces mainly hydrocinnamaldehyde under the same reaction conditions. Compared to the conventional organic solvents, improved activity and selectivity has been observed for both the catalysts in scCO(2) medium. The results suggested a strong influence of oxidation state and particle size along with the beneficial role of scCO(2) on the hydrogenation of cinnamaldehyde using Pt catalysts obtained by the different synthetic methods. (C) 2003 Elsevier B.V. All rights reserved.