The adsorption of hydrogen and carbon monoxide on MgO and CaO have been studied at ambient temperature using u.v.–visible diffuse reflectance spectroscopy. Hydrogen affects the surface spectrum of both oxides, but the influence is more pronounced with MgO, where different adsorption modes evidently coexist. CO adsorption on MgO and CaO produces polymeric anions, and interaction with hydrogen has enabled the two cyclic oxocarbon anions, croconate (C₅O^2–₅) and rhodizonate (C₆O^2–₆), to be characterized separately. The reflectance results show that both are present on each oxide, but croconate dominates on CaO and rhodizonate on MgO. The reaction of rhodizonate dianion with hydrogen at room temperature has been followed spectroscopically. As C₆O^2–₆(which absorbs at 21 500 cm^–1 on MgO) reacts, a new species absorbing to give a broad band with a maximum at 29000 cm^–1 is generated. The reaction is identified as a surface protonation producing the monoanion HC₆O^–₆. Croconate dianion is unreactive towards hydrogen. The relative abundances of croconate and rhodizonate on MgO and CaO and the variations in reactivity are discussed with reference to the differences in basicity between the two oxides.