The effects of TiO₂ addition on the sintering process and microstructure of alumina ceramics were studied. In air, the solid solubility of TiO₂ in α-Al₂O₃ was too small to be determined by the lattice parameter shift of α-Al₂O₃. Then, the relative amounts of titanium compounds remaining in fired bodies were measured by X-ray diffractometry using a step-scanning technique which can detect less than 0.1wt% rutile or Al₂TiO₅ in α-Al₂O₃, and were compared with the amount of TiO₂. The solid solution of TiO₂ in α-Al₂O₃ was found above 1150°C, and the solubility was estimated to be 0.27wt% at the temperature range from 1300° to 1700°C. Beyond the solubility limit, excess TiO₂ coexisted with α-Al₂O₃ as rutile below 1350°C and as Al₂TiO₅ above 1450°C. The sintering of α-Al₂O₃ was markedly promoted when TiO₂ was added up to the solubility limit and the fired density higher than 97% of the theoretical was obtained at 1400°C. The addition of TiO₂ also promoted the grain growth of α-Al₂O₃. But beyond the solubility limit, the grain size decreased with an increase of Al₂TiO₅. Therefore it is inferred that Al₂TiO₅ existing as a second phase retards the grain growth of α-Al₂O₃. The lattice parameters of Al₂TiO₅ in fired bodies considerably differed from the those of a single crystal. It is explained as due to the difference of thermal expansion coefficient between Al₂TiO₅ and α-Al₂O₃.