The basicity of oxide surfaces is often associated with surface lattice oxygen anions. CO2 is the standard probe molecule for investigating surface basicity, but limited information is available in the surface science literature concerning its interaction with well-defined (single crystal) oxide surfaces. On stoichiometric. Cr2O3 (10 (1) over bar2), CO2 interacts with cation/anion site pairs to form bidentate carbonates that are stable at room temperature. This site pair consists of five-coordinate Cr3+ cations and three-coordinate O2- anions on the clean, stoichiometric, nonpolar (10 (1) over bar2) surface. Terminating the surface cations with chromyl oxygen (Cr=O) via dissociative O-2 chemisorption breaks the interaction and gives rise to a weakly bound CO2 moiety, tentatively identified as a "monodentate" CO2 adsorbate bound to terminal chromyl oxygen sites. Differences in the coordination of these species make the heats of adsorption a poor measure of the basicity of surface oxide ions. Terminating the surface cations with chlorine adatoms blocks the interaction between CO2 and the surface, and no uptake is observed.