The processes of phase formation in the Nd₂O₃-Ho ₂O₃-SrO-Al₂O₃ system are investigated in the temperature range 1100-1530°C. It is revealed that the structural-chemical mechanism of formation of (Nd_xHo _1-x)₂SrAl₂O₇ solid solutions depends on the temperature and composition. An increase in the holmium content and in the temperature leads to a crossover from the mechanism including the formation of LnAlO₃ and LnSrAlO₄ as intermediate products to the mechanism in which the interaction of Ln₂O₃ and SrAl ₂O₄ is a limiting stage.