Ethene adsorption on clean and oxygen-modified Cu(110) surfaces was investigated by infrared reflection absorption spectroscopy (IRAS). The end-on species of adsorbed ethene, as was reported by Jenks et al., was observed on a clean Cu(110) surface. It was, however, suggested that the adsorbed ethene structures at higher temperature were different from those obtained at lower temperature. On oxygen-modified Cu(110) surfaces, end-on ethene decreased with the increase of the amount of surface oxygen. The pi-adsorption occurred only when the (2 X 1)-O/Cu(110) reconstructed surface was formed. The pi-bonded ethene on the (2 x 1)-O/Cu(110) adsorbed reversibly even at 110 K, and the heat of adsorption was estimated to be 32 +/- 7 kJ/mol from the analysis of the adsorption isobar obtained by IRAS.