The phase equilibria are investigated and the phase diagram is constructed for the LaAlO₃-LaSrAlO₄ pseudobinary join of the La ₂O₃-SrO-Al₂O₃ ternary oxide system. One ternary compound, namely, La₂SrAl₂O₇, is found in the LaAlO₃-LaSrAlO₄ join. The crystallographic, thermal, and some other physicochemical characteristics of the compounds are reported.