Interactions of both liquid and vapor water with stoichiometric (nearly defect-free) and defective TiO 2 (100) surfaces have been studied using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). For an almost defect-free (100)1x1 surface, water coverage was ~0.08ML (1ML=7.36x10 1 4 /cm 2 ) at 10 4 L exposure to low-vapor-pressure water, ~0.32ML at 10 8 L exposure to higher-vapor-pressure water, and ~0.50ML with liquid-water exposure, respectively. Defect intensities were greatly reduced after exposing defective surfaces to ~10 2 L low-vapor-pressure water. More significantly, electron-beam-induced defects were completely removed upon higher exposure (>10 4 L), while defects created by Ar + bombardment were partially removed. The surface structural influence on defect reactivity has been examined by comparing the results obtained from both (100) and (110) surfaces. Defects on (100) surfaces were removed more readily than those on (110) surfaces.