The infrared spectra of CO adsorbed at 77 K on sintered polycrystalline MgO and Ni/MgO, Co/MgO solid solutions, containing 10% nickel and cobalt in atoms are reported. These solids are properly designed for the estimation of the role played by d–π overlap in the bonding between the isolated transition-metal cations and the CO molecules adsorbed on them. In fact, since the Ni²⁺ or Co²⁺ ions emerging on the (100) faces are diluted in a common MgO matrix, they experience an environment and an ionicity similar to that of the Mg²⁺ ions. Consequently any difference in the CO stretching frequency can principally be ascribed to overlap contributions. Moreover, the nature of the lateral interactions between Ni²⁺—CO and Co²⁺—CO complexes and the surrounding (predominant) Mg²⁺—CO species can be investigated and compared with those observed on the pure oxides. The examination of the experimental results on such model solids confirms that, although the interaction of CO with Ni²⁺ and Co²⁺ ions is mainly electrostatic, some d–π overlap contribution is present.