Vapor-phase selective hydrogenation of crotonaldehyde was carried out over Ir/TiO 2 catalysts with different Ir contents prepared by an impregnation method. The catalysts were characterized by X-ray powder diffraction (XRD), temperature-programmed reduction (TPR), diffuse reflectance infrared Fourier transform spectra of CO adsorption (CO-DRIFTS), NH 3 temperature-programmed desorption (NH 3 -TPD), Raman spectroscopy and temperature-programmed oxidation (TPO). It was found that with increasing Ir content in Ir/TiO 2 , both the activity (TOF) and selectivity to crotyl alcohol first increased and then slightly decreased. The 3 % Ir/TiO 2 catalyst showed the highest activity, (with a TOF of 9.3 × 10 −3  s −1 ) and the highest selectivity to crotyl alcohol (74.6 %) in the hydrogenation of crotonaldehyde. The results of CO-DRIFTS indicated that the reduced catalyst contains a mixture of Ir 0 and Ir δ+ . It was concluded that the catalytic performance of the catalysts depended on the strength of surface acidity and the Ir particles size for the selective hydrogenation of crotonaldehyde to crotyl alcohol.