We have examined the interaction of molecular oxygen with the TiO2(110) surface using temperature-programmed desorption (TPD), isotopic labeling studies, sticking probability measurements, and electron energy loss spectroscopy (ELS). Molecular oxygen does not adsorb on the TiO:(110) surface in the temperature range between 100 and j00 K unless surface oxygen vacancy sites are present. These vacancy defects are generated by annealing the crystal at 850 K, and can be quantified reliably using water TPD. Adsorption of O-2 at 120 K on a TiO2(110) surface with 8% oxygen vacancies (about 4 x 10(13) sites/cm(2)) occurs with an initial sticking probability of 0.5-0.6 that diminishes as the surface is saturated. The saturation coverage at 120 K, as estimated by TPD uptake measurements, is approximately three times the surface vacancy population. Coverage-dependent TPD shows little or no Oz desorption below a coverage of 4 x 10(13) molecules/cm(2) (the vacancy population), presumably due to dissociative filling of the vacancy sites in a I:1 ratio. Above a coverage of 4 x 10(13) molecules/cm(2), a first-order O-2 TPD peak appears at 410 K. Oxygen molecules in this peak do not scramble oxygen atoms with either the surface or with ether coadsorbed oxygen molecules. Sequential exposures of O-16(2) and O-18(2) at 120 K indicate that each adsorbed O-2 molecule, irrespective of its adsorption sequence, has equivalent probabilities with respect to its neighbors to follow the two channels (molecular and dissociative), suggesting that Oz adsorption is not only precursor-mediated, as the sticking probability measurements indicate, but that all O-2 molecules reside in this precursor state at 120 K;. This precursor state may be associated with a weak 145 K O-2 TPD state observed at high Oz exposures. ELS measurements suggest charge transfer from the surface to the Oz molecule based on disappearance of the vacancy loss feature at 0.8 eV, and the appearance of a 3.8 eV loss that can be assigned to an adsorbed O-2(-) species based on comparisons with Ti-O-2: inorganic complexes in the literature. Utilizing results from recant spin-polarized DFT calculations in the literature, we propose a model when three O-2 molecules are bound in the vicinity of each vacancy site at 120 K. For adsorption temperatures above 150 E;, the dissociation channel completely dominates and the surface adsorbs oxygen in a 1:1 ratio with each vacancy site. ELS measurements indicate that the vacancies are filled, and the remaining oxygen adatom which is apparent in TPD, is transparent in ELS. On the basis of the variety of oxygen adsorption states observed in this study, further work is needed in order to determine which oxygen-related species play important roles in chemical and photochemical oxidation processes on TiO2 surfaces.