Titanium dioxide is widely used as support for various important catalysts. Although nonstoichiometric titania behaves as an n-type semiconductor, the nature of the defect sites is not yet fully understood. In the present investigation the water vapor adsorption and carbon monoxide oxidation on TiO2 is explained considering oxygen vacancies as the major defect. It is also shown that incorporation of an Al3+ ion in TiO2 reduces the concentration of oxygen ion vacancies and inhibits the transformation of anatase to rutile.