Reactions between titanium and alumina were studied experimentally for Al2O3 substrates with a titanium-based coating and for planar Ti-Al2O3 diffusion couples in the temperature range between 800 and 1100°C. Isothermal sections through the phase diagram were determined by using these results as well as by investigating equilibrated alloys. These experimental sections agree with those calculated from thermodynamic data. The morphology and layer thickness of the observed reactions zones have been explained on the basis of these phase relations, making use of diffusion data from binary systems Ti---O and Ti-Al. The important role of the initial Ti thickness on the type of reaction products has been demonstrated.