Infrared spectra of adsorbed CO have been used to monitor the surface of Cu/TiO₂ which had been reduced in H₂ at 523, 623 or 723 K and was also exposed to propene, butanal or crotonaldehyde. For CO alone infrared bands at 2118–2133 cm⁻¹ dominated the spectra and are ascribed to CO interacting with Cu(I) sites in chains or two-dimensional rafts of Cu atoms distributed at lattice points determined by the geometric and electronic character of the underlying TiO₂ surface. Three effects of crotonaldehyde on co-adsorbed CO are a geometric site blocking effect which reduces the surface concentration of sites available for CO adsorption, an electronic effect due to the electron-donating character of crotonaldehyde acting as a Lewis base and a redox effect which leads to the oxidation of crotonaldehyde to adsorbed crotonate ions and the reduction of Cu(I) to Cu(0). The enhanced selectivity, induced by TiO₂ as support, towards crotyl alcohol rather than butanal formation from crotonaldehyde hydrogenation over Cu is attributed to the promotion by TiO₂ of Cu(I) sites and the co-existence on catalyst surfaces of Cu(0) and Cu(I) sites which are particularly active for hydrogenation of carbonyl (CO) bonds.