When alpha,beta-unsaturated aldehydes are reduced by hydrogen in supercritical carbon dioxide using an unmodified Pt/Al2O3 catalyst, unsaturated alcohols are highly selectively produced, in contrast to previous results with organic solvents in which such a selective hydrogenation is difficult to achieve with monometallic Pt catalysts. The selectivity of unsaturated alcohols depends mainly on pressure of carbon dioxide while the conversion depends on both carbon dioxide and hydrogen pressures. The selectivity increases with increasing pressure and then does not change so much at higher pressures.