Phase relations in the MgO-TiO2 and Al2O3-MgO-TiO2 systems have been studied experimentally using X-ray diffraction, scanning electron microscopy combined with dispersive X-ray spectrometry, and differential thermal analysis. The heat capacity measurements of the Mg2TiO4, MgTiO3, and MgTi2O5 compounds have been carried out in the temperature range of 473-1373K using DSC. A new thermodynamic description of the MgO-TiO2 system has been derived based on obtained experimental results, data from literature, and taking into account the cation disordering of the intermediate compounds Mg2TiO4 and MgTi2O5. The liquid phase was described by the two-sublattice partially ionic model (Mg+2,Ti+2,Ti+3)(O-2,Va,O,TiO2). For the Al2O3-MgO-TiO2 system: the solid-state invariant reaction involving the intermediate phase of pseudobrookite solid solution have been found at the temperature of 1433K. On the liquidus surface, the eutectic invariant reaction has been established at 1843K and the transitional oneat 2006K. Based on the obtained experimental results isothermal sections within the range 1269-1697K of the ternary system have been predicted. The MgTi2O5-Al2TiO5 section has been constructed.