Temperature-programmed desorption (TPD) and oxygen isotopic labeling studies were used to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile). Water TPD spectra from these two surfaces were distinctive. Three monolayer desorption states were observed for the (100) surface (at 205, 250, and 305 K), while only a single desorption state was observed for the (110) surface (at 270 K). TPD experiments on the surfaces enriched with O-18 revealed that water desorbing in the 305 K state from TiO2(100) was isotopically scrambled with the lattice oxygen atoms, strongly suggesting that this TPD state resulted from recombination of surface hydroxyl groups. Isotopic scrambling was not observed for any other desorption state on either surface (in the absence of defects). Since very little water desorption occurred from the (110) surface in the temperature range in which exchange was observed on the (100) surface and since previous HREELS work (Henderson, M. A. Surf : Sci. 1996, 355, 151) indicated that very little water dissociation was detected for the (110) surface, the ideal TiO2(110) surface appears to be inactive for water dissociation under ultrahigh vacuum (UHV) conditions. Comparison of the geometric arrangement of acidic and basic sites on these two surfaces suggests that the bridging two-coordinate O2- sites (basic sites) on TiO2(110) are too distant from the binding sites of water (five-coordinate Ti4+ sites) to form hydrogen-bonding interactions with water that might facilitate O-H bond dissociation, whereas the proximity of these sites on the TiO2(100) surface should favor such a concerted interaction. The TiO2(110) surface was active for dissociation of water when structural defects such as steps or kinks were present. Defects created by annealing or by electron-beam irradiation were less active for water dissociation.