Adsorption of ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O on CaO and MgO, pretreated at 1000 K, has been studied at 40-300 K. A number of surface CO compounds have been detected, which differ in their thermal stability or sensitivity with respect to oxygen. From the measured isotopic shifts and frequencies the force constants of some compounds were calculated. The first product arising after CO admission is the "carbonite" CO₂ ^2- ion, which is the result of CO interaction with a coordinatively unsaturated oxygen ion. At temperatures higher than 100 K the carbonite ion reacts with excess CO to produce the dioxoketene OCCO₂ ^2- ion. Further reactions with CO give rise to more complex compounds, both oxidized and reduced. In the presence of oxygen, carbonite ions and reduced CO compounds are readily oxidized into carbonates; at low temperature, some intermediate products of oxidation were registered. Reversible interaction of CO with the surface O^2-ions is responsible for the catalytic reaction of low-temperature homomolecular isotopic exchange of CO on the surface of basic oxides. It is supposed that the established mechanism of CO activation by oxygen ions is typical for other reactions on the surface of basic and alkali-promoted catalysts.