The behavior of H2O on clean and trimethyl acetate (TMA)-covered TiO2(110)-(1 X 1), prepared with or without oxygen vacancies and associated Ti3+, reveals the hydrophilic nature of clean surfaces and the hydrophobic nature of TMA-covered surfaces. UV irradiation of a hydrophobic surface in the presence of 10(-6) Torr of O-2 removes TMA and rapidly restores hydrophilicity. The presence of oxygen atom vacancies does not delectably alter the hydrophilicity of either clean or TMA-covered TiO2(110).