Insight in the interplay between work function and stability is important for many areas of physics. In this paper, we calculate the anisotropy in the work function and the surface stability of CrO2, a prototype half-metal, and find an anisotropy of 3.8 eV. An earlier model for the relation between work function and surface stability is generalized to include the transition-metal oxides. We find that the lowest work function is obtained for surfaces with the most electropositive element, whereas the stable surfaces are those containing the element with the lowest valency. Most CrO2 surfaces considered remain half-metallic, thus the anisotropy in the work function can be used to realize low resistance, half-metallic interfaces.