The join MgSiO₃-MgAl₂SiO₆ was studied with the special reference to the solubility of Al₂O₃ in enstatite, above 1300°C. MgAl₂SiO₆ is incorporated in enstatite as much as 3.5wt.% (1.75wt. % Al₂O₃) at atmospheric pressure. At subsolidus temperatures the phase assemblage protoenstatite_ss+forsterite was confirmed in the composition ranging from 96.5 to 94.5wt. % MgSiO₃. In the more aluminous region the subsolidus phase as-semblage is protoenstatite_ss+forsterite+cordierite_ss. The experimental result indicates that the composition of protoenstatite lies not only on the join MgSiO₃-MgAl₂SiO₆ but also has a solid solution area on the MgO-Al₂O₃-SiO₂ plane as shown in Fig. 4. The unit-cell dimensions of the protoenstatite_ss crystallized at 1300°C were determined. a, b, and V decrease with increasing Al₂O₃ contents in the protoenstatite_ss. The application of the results to meterorite as well as terrestrial rock is discussed. The Al₂O₃ contents of enstatite in meteorite may be used as an indicator of temperature scale.