Accurate first-principles calculations are used to study the relaxed stoichiometric and reduced SnO 2 (110) surfaces. The reliability of the calculations is supported by tests on the perfect crystal. Sizable atomic relaxations at both surfaces are found. Oxygen loss leaves electron density around surface Sn atoms, which can be regarded as being in the Sn 2+ state. There are gap states for the reduced but not for the stoichiometric surface.