Phase formation in the La₂O₃-SrO-Al₂O₃ system was studied. Formation of the perovskite LaAlO₃ structure and the perovskite-like LaSrAlO₄ and La₂SrAl₂O₇ layered structures in solid-phase chemical reactions is activated only after reaching certain critical temperatures which correlate with the temperatures of of the transition of intergranular formations to the liquid-like state (melting points of two-dimensional nonautonomous phases).