The interaction of Ti with A12O3 under UHV conditions has been studied by AES and UPS. Ti was deposited by iterated and successively thicker evaporations (up to a total thickness of 91 Å) under UHV conditions onto the alumina substrate at room temperature. The oxide substrate was grown on Al/Si(111) in an adjacent VHV preparation chamber. The last deposition was followed by in situ annealing. It was found that, contrary to indications of thermodynamic considerations using bulk equilibrium data, Ti interacts strongly with alumina at room temperature, dissociating it to metallic Al and forming TiO2 at the interface. This reaction is limited, resulting in an interface width of ∼10 Å and allowing subsequent evaporations to homogeneously cover the products. Annealing to 500°C does not produce any evidence of outdiffusion or extensive interface reactions, implying that the interface oxide is stable and an effective barrier to Al (and/or Ti) diffusion up to this temperature.