Ab initio spin-unrestricted periodic Hartree-Fock calculations have been used to study the electron-excess gap states in rutile TiO 2 which result from the formation of oxygen vacancies in the bulk and at the surface of an isolated slab of infinite {110} planes. These states are found to be insulating and occur at 1-2 eV above the O 2p band edge. It is shown that while the O 2p band undergoes small changes in both width and shape as a result of these defects, it remains essentially spin-paired. On the other hand, gap states associated with these defects are largely spin-polarised states corresponding to excess electron density trapped at bulk vacancies and local Ti 3d 1 and Ti d 2 configurations in both the bulk and at the surface. In the bulk the lowest energy state is one in which the majority of the excess charge is localised at the vacant oxygen site, whereas at the surface the redistribution is essentially at the surface Ti atoms. For the oxygen-deficient {110} surface with an entire plane of missing oxygen atoms, an antiferromagnetic spin arrangement is found consisting of localised spins on the surface titanium atoms.