Acidic and basic properties of lanthana, pretreated at 500-1200 K, have been investigated by means of CO adsorption at 100-300 K. ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O molecules were used as well as adsorption on La₂O₃ enriched with ¹⁸O. At low temperature, molecular adsorption on weak Lewis acid sites occurs. Study by the isotopic dilution method shows that both static (Δv_st = 14 cm^-1) and dynamic (Δv_dyn = 8 cm^-1) effects cause the observed CO band shift with the increasing coverage. Surface OH groups, which account for the narrow bands at 3683 and 3662 cm^-1, are not acidic enough to adsorbed molecular CO at 100 K; at 300 K the high-frequency hydroxyls react with CO forming formate ions. Basic properties of the dehydroxylated surface manifest themselves when the first doses of CO are chemisorbed on La₂O₃ pretreated at 800 or 1200 K: the reaction with surface oxygen ions produces "carbonite" CO₂ ^2- ions characterized by bands at 1396, 983, and 760 cm^-1. This form of adsorption is irreversible and insensitive with respect to oxygen admission at 100 K, while at 300 K it can be removed by evacuation or oxidized by oxygen with the formation of surface carbonates. Reversible CO adsorption in the form of carbonite ions at 300 K is accompanied by oxygen isotope exchange. The concentration of the surface O^2- sites responsible for CO₂ ^2- ion formation is about 1.1 nm^-2.