We have carried out a computational bulk and surface study of the behavior of Sb(III) and Sb(V) ions on the (110) and (001) surfaces of SnO2. In addition, we have also examined the behavior of the Sn(II) and oxygen vacancy complex in the bulk and surface. These calculations suggest that Sb(III) is associated with in-plane surface oxygen species, while Sb(V) is subsumed below bridging oxygen ions in a more bulk-like environment. In addition, we find two possible Sn(II)/O-vacancy complex sites for the (110) surface: one is associated with a bridging oxygen vacancy, and the most favorable arrangement is associated with an in-plane subsurface oxygen vacancy. These calculations indicate not only the most favorable complex sites but also predict the surface segregation and defect energies. The theoretically derived sites are in complete agreement with the experimental data reported recently and proposed models for sensor activity.(1,2).