Infrared spectra have been recorded of solutions in liquefied argon (85-115 K) containing both boron trifluoride and carbon monoxide. The formation of complex species was concluded from the appearance of bands not attributable to the monomer molecules. Using concentration variation, bands due to the 1:1 complex BF3 . CO and to the 1:2 complex BF3 .(CO)(2) have been identified. From temperature-dependant studies, the enthalpy of formation of the 1:1 complex was found to be -7.6 +/- 0.3 and - 14.5 +/- 1.0 kJ mol(-1) for the 1:2 complex. A very weak band, presumably due to a third complex, is tentatively assigned to the 1:1 complex BF3 . OC in which CO is binding to the boron atom via its oxygen atom. The solvent shifts of upsilon(2) and upsilon(3) of BF3 and the complexation shifts of these modes are interpreted in terms of a simple induction model.