Acid/base characterizations of metal oxide surfaces are used often to explain their catalytic behavior. BF3, a strong Lewis acid, is used in this investigation as a probe molecule to interrogate the basicity of well-defined Cr2O3 (10 (1) over bar2) surfaces. BF3 clearly probes differences in oxide ions of different coordination on Cr2O3 (10 (1) over bar2). The heat of adsorption of BF3 shows that terminal chromyl oxygens are stronger Lewis bases than three-coordinate oxygen anions on the nearly stoichiometric surface. The trends in basicity found with BF3 are the opposite of those found from the heat of adsorption Of CO2, a,common probe molecule used to investigate the basicity of oxide surfaces. The use of BF3 as a probe molecule is complicated by some dissociation and the,slow build up of surface boron and fluorine during consecutive thermal desorption runs.