Hydrogenation of a,b-unsaturated aldehydes (citral, 3-methyl-2-butenal, cinnamaldehyde) has been studied with tetrakis(triphenylphosphine) ruthenium dihydride (H 2 Ru(TPP) 4 ) catalyst in a poly(ethylene glycol) (PEG)/compressed carbon dioxide biphasic system. The hydrogenation reaction was slow under PEG/H 2 biphasic conditions at H 2 4 MPa in the absence of CO 2 . When the reaction mixture was pressurized by a non-reactant of CO 2 , however, the reaction was significantly accelerated. As CO 2 pressure was raised from 6 MPa to 12 MPa, the conversion of citral increased from 35% to 98%, and a high selectivity to unsaturated alcohols (geraniol and nerol) of 98% was obtained. The products were able to be extracted by high pressure CO 2 stream and separated from the PEG phase, dissolving the Ru complex catalyst and the catalyst was recyclable without any post-treatment.