Infrared spectra were obtained for Ar/CO samples codeposited with laser ablated boron atoms on a 12 +/- 1 K salt window. BCO was identified as the primary reaction product, based on isotopic substitution at all atomic positions and ab initio structure and frequency calculations. The dicarbonyl, B(CO)2, and the linear dimer, (BCO)2, were also identified from the infrared spectra. A higher cluster species containing bridging carbonyls appeared on annealing and was stable until the matrix sublimed away.