Co-deposition of argon or nitrogen dilute samples of natural and isotopic carbon monoxide and halogen molecules (Cl-2, Br-2, BrCl) produced new narrow absorptions:lines assigned to X-2... CO complexes. With BrCl, in an argon matrix two 1:1 complexes were observed, one stronger than the Br2 CO complex, the other one weaker than the Cl-2... CO complex. In the nitrogen matrix, one complex was only observed, namely, the ClBr .. CO complex. For all complexes, observed shifts were larger in nitrogen than in argon matrices and similar to available gas-phase results for Cl-2... CO and Br-2... CO complexes. Irradiation of the samples with a filtered xenon lamp (lambda > 360 nm) induced in an argon matrix (not in a nitrogen matrix) the isomerization of the complexes into a metastable form CO ... X-2 identified for the first time. Back isomerization was observed by weak temperature increase. The structure and energetics of the four complexes formed between CO and BrCl were investigated by DFT ab initio calculations and compared to complexes formed between CO and Cl-2 and Br-2.