Weakly bound 1: 1 complexes formed between CO and a dihalogen molecule XY = Cl-2, BrCl, ICl, or IBr have been investigated by matrix-isolation experiments and density functional theory (DFT) calculations. Two families of these linear complexes exist according to whether the C or the O atom of the CO binds to the dihalogen, and with a heteronuclear dihalogen XY, this binding may occur to either of the dihalogen atoms, giving four possible isomeric forms. In addition to the nu(CO) modes of the various complexes, IR measurements have identified nu(XY) modes for the complexes of the type OC...XY. The structures, vibrational properties, and binding energetics of the complexes are analyzed in light of related studies.