Infrared spectra of propionaldehyde was studied over silica-supported Rh−Sn bimetallic catalysts. Two absorption bands of the carbonyl group were observed at 1670 and 1720 cm-1, and the aldehyde hydrogen (−CHO) was also observed at 2748 and 2848 cm-1 over the Sn/Rh/SiO2 catalysts on which propionaldehyde was preadsorbed. One of the absorption bands of carbonyl groups at 1670 cm-1 readily disappeared by contact with H2, whereas the other band at 1720 cm-1 remained. The band of the aldehyde hydrogen also disappeared by the contact with H2. The absorption band at 1670 cm-1 was assigned to a donating-on-top η adsorbed species that were bound to tin atoms with oxygen atoms of carbonyl groups. The band observed at 1720 cm-1 was assigned to a species weakly adsorbed on the catalyst surface. The intensity of the band at 1670 cm-1 was increased with an Sn/Rh ratio up to unity and then was gradually decreased with the Sn/Rh ratio of the catalysts. The reduction temperature at 573 K yielded a maximum intensity ratio of I 1670/I 1720. The activity for selective hydrogenation of unsaturated aldehydes to unsaturated alcohols indicated a maximum at 573 K of reduction temperature. These results clearly indicate that the adsorbed species observed at 1670 cm-1 in the infrared spectra is one of the most effective surface adsorption states in the selective hydrogenation reaction of carbonyl groups in unsaturated compounds. Adsorption experiments indicated that the tin addition increased the O2-adsorption capacity of the catalysts. Tin showed a high affinity for oxygen. These results can conclude that one of the roles of tin is selective activation of carbonyl groups by binding to the Sn atoms with oxygen atoms of carbonyl groups in unsaturated carbonyl compounds, which can be reduced to unsaturated alcohol by hydrogen atoms spilt over from Rh sites, in the hydrogenation of unsaturated aldehydes.