The reactivity of rutile phase TiO2 (1 1 0) and polycrystalline anatase surfaces with molecular and ionized deuterium and hydrogen was investigated. Thermal Desorption Spectroscopy (TDS), following exposure to more than 100 kL of molecular deuterium, showed a D-2 desorption peak at similar to440 K on both single crystal rutile and polycrystalline anatase surfaces. The desorption peak was observed following exposure only at surface temperatures between 140-270 K. Ionized D-2 is significantly more reactive with the titania surface and two desorption peaks at 380 K and similar to550 K were observed together with a small D2O peak observed at similar to440 K. Dosing the surfaces with hydrogen and deuterium either in succession or as a mixture showed HD desorption with no change in the desorption peaks, consistent with dissociative adsorption of both ionized and molecular species. The experimental data was compared to Density Functional Theory calculations and modeled as a two-step process of hydrogen dissociation at oxygen vacancy sites on TiO2. (C) 2004 Elsevier B.V. All rights reserved.