Characterization of CO species adsorbed on well-degassed surfaces of polycrystalline MgO at low CO pressures (< 1300 Pa) and at and below room temperature was studied by using temperature-programmed desorption (TPD) and infrared (LR) spectroscopies. Mutual transformation between adsorbed species stable at room temperature, and their reactivities with O-2 were also investigated. Adsorbed species are classified into five groups, K-0, K-1 (and K-1’), K-2, K-3, and K-4. Among them species K-0 and K-1 (and K-1’) are stable only below room temperature while species K-2 to K-4 are stable above this temperature. Species K-0 is a linear-type CO monomer with the C atom bonded to surface Mg2+ and has five subspecies depending on its circumstance, while all the other species K-1 to K-4 are formed on surface O2- to which the C atom is linked. A chained-type CO monomer, K-1, is reversibly transformed at ca. 230 K in the presence of gaseous CO into a linear-type CO monomer, K-1’. Species K-2 is a chained-type tetramer of CO with a carbonyl CO bond and resonanced bonds. There are two subspecies, K-2A and K-2X, which are formed by the addition of one CO molecule to corresponding trimer species, K-3A and K-3X, respectively, in the presence of gaseous CO. K-2A and K-2X are reversibly transformed into K-3A (at 400 K) and K-3X (in 300-400 K), respectively, by the elimination of the CO added. There are two more subspecies, K-3B and K-3C, in species K-3, but they have no corresponding K-2 subspecies, and hence no transformations occur. All the K-3 species seem to be a linear-type trimer with a ketenic group, > C=O=O, and are desorbed in 520-580 K. Species K-4, though detailed information is not obtained, consists of three subspecies K-4A, K-4B, and K-4C, but no transformations between species K-3 and K-4 are observed.