Infrared spectra of the CO2-CO dimer are observed in the carbon monoxide CO stretch region (approximate to 2150 cm(-1)). Combination bands yield the first experimental determinations of intermolecular mode frequencies for the planar T-shaped C-bonded form ('isomer 1'): 24.34 cm(-1) for the in-plane CO rock and 43.96 cm(-1) for the out-of-plane CO rock. These values are significantly higher than the analogous modes of the O-bonded form ('isomer 2'), previously determined to be 14.19 and 22.68 cm(-1), respectively. New results are also reported for both isomers of the (CO2)-C-12-O-18-substituted form of the dimer. Weak 'satellite bands' observed for both isomers are tentatively assigned to the trimer He-CO2-CO. (C) 2016 Elsevier B.V. All rights reserved.