The ground-state rotational spectra of three isotopomers, 12C 16O 2⋯ 35ClF, 12CO 2⋯ 37ClF and 13CO 2⋯ 35ClF, of a complex formed by carbon dioxide with chlorine monochloride have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The following values of the spectroscopic constants B 0, D 0, H 0, χ aa (Cl) and M bb (Cl) have been determined for this linear (or nearly linear) molecule. 12C 16O 2⋯ 35ClF 12CO 2⋯ 37ClF 13CO 2⋯ 35ClF B 0 (MHz) 916.1184(2) 909.7692(2) 906.2644(5) D 0 (kHz) 5.435(5) 5.381(6) 5.31(2) H 0 (Hz) 0.37(3) 0.34(5) 0.4(1) χ aa (Cl) (MHz) −136.804(4) −107.812(5) −136.820(8) M bb (Cl) (kHz) −2.5(4) −0.9(4) −3.4(5) Interpretation of the spectroscopic constants reveals that the nuclei lie in the order OCO⋯ClF, with r(O⋯Cl)=2.699(2) Å, and that the intermolecular bond is very easily distorted both radially and angularly. A comparison of the series B⋯HF and B⋯ClF for B=Ar, N 2, CO or CO 2 establishes that CO 2⋯ClF is anomalous in respect of the strength of its binding.