The complexes formed by isothiocyanic acid HNCS and its deuterated analogue DNCS with carbon monoxide have been observed and characterized in argon and nitrogen matrixes. The product bands and their shifts relative to the corresponding monomer absorptions prove that the complexes are hydrogen bonded with the carbon-attached structure OC...HNCS. The structure, energetics, and vibrational properties of the complexes have been calculated by ab initio at the MP2 level. Two stable minima were localized on the potential energy surface. Both involved an almost linear hydrogen bond from the NH group of the isothiocyanic acid molecule to either the carbon or the oxygen atom of the CO molecule. The OC...HNCS and CO...HNCS complexes were found to be weakly bound by -2.84 and -1.33 kcal/mol, respectively.