ClCO was produced as a reaction intermediate after irradiation of a flowing mixture of Cl-2/CO/Ar at 355 nm. A step-scan time-resolved Fourier-transform infrared spectrometer operated in absorption mode was employed to detect ClCO. A transient spectrum at a resolution of 0.13 cm(-1) partially reveals rotational structure with the Q-branch peaked at 1884.59 cm(-1); the transition is associated with the C-O stretching (nu (l)) mode of ClCO. Calculations with densityfunctional theory (B3LYP/aug-cc-pVTZ) predict the geometry, vibrational and rotational parameters of (ClCO)-Cl-35 and (ClCO)-Cl-37. A simulated absorption spectrum based on these parameters agrees satisfactorily with experimental observation.