The adsorption of CO on microcrystalline MgO and CaO has been studied by u.v.–visible and i.r. spectroscopies. U.v.–visible absorption is generated by CO-derived species in two distinct bands, namely a ‘high-v’ absorption at 28 000–35 000 cm^–1, and a ‘low-v’ absorption at 18 000–25 000 cm^–1. The species contributing to these respective absorptions have been discriminated by observing selective reaction with O₂, N₂O and propene. The low-v absorption is due to reduced polymeric species, viz. cyclic anion clusters (CO)^2–_n, and the high-v absorption to lower order non-reduced species incorporating surface oxide ions, with (CO)_nO^2– stoichiometry. There are marked differences in behaviour between CaO and MgO. I.r. studies using ¹³C- and ¹⁸O-labelled CO show that, under conditions where high-v absorption develops in the u.v. on CaO, vibrational spectra are generated which are characteristic of a C₁ species akin to a metallated formate [Ca²⁺…(CO₂)^2–], with oxygen atoms which, although non-equivalent, undergo rapid exchange. The high-v u.v. absorption on CaO is therefore ascribed to this species. With MgO, on the other hand, experiments using propene as a probe show the high-v u.v. absorption in that case is mainly due to higher-order species, notably the (CO)₃O^2– ketenic entity (OCCRR′)^2–, with R = O^– and R′= O^–—CO. Although these non-reduced species are stable as such at many surface locations, certain sites are sufficiently reactive to allow them to undergo reaction with CO to yield the cyclized clusters (CO)^2–_n(especially n= 5 or 6) and concomitant formation of CO^2–₃. These reduced clusters are reactive towards oxygen and N₂O, but not towards propene. By contrast, the simpler non-reduced (CO)_nO^2– species are shown by i.r. to couple with propene, yielding ultimately alkoxide and carboxylate.