Abstract SHG studies on polished and etched (110) rutile TiO 2 single-crystal surfaces suggest that above band gap, low-energy (4.7 eV) ultraviolet photons create stable Ti 3+ surface defects in UHV. Such defects can be healed by subsequently exposing the surface to O 2 gas. These results are similar to recently reported measurements on polished and etched (001) rutile single-crystal surfaces. Observations on the (001) surfaces were interpreted as the photodesorption and re-adsorption of molecular oxygen bound loosely to Ti 3+ defects on the surface as a Ti 4+:O − 2 complex. For the current (110) study, XPS was used to confirm the defect type and to quantify the density of Ti 3+ defects created. Defects on the surfaces studied using XPS were generated using photons of even lower energy (3.4 eV), indicating that the oxygen species removed was very loosely bound. The same defect creation and healing processes were also observed on a nearly defect-free thermally annealed single-crystal surface using XPS. Whether bridging oxygen ions on the (110) surface or O − 2 ions are the photo-labile species has yet to be determined. Transient photodesorption of O 2 at high oxygen pressure also was observed using SHG. SHG has proven to be a sensitive probe of surface defects, consistent with XPS results, under conditions ranging from UHV to atmospheric.