Cation distribution and interatomic distances in rare-earth alkaline-earth aluminates of the K₂NiF₄- and Sr₃Ti₂O₇-type layered structures was studied. The instability of the layered structure in La, Ca-containing compounds was explained from the crystal-chemical and thermodynamic viewpoints. For LaCaAlO₄, the structure-chemical decomposition mechanism was proposed. The effect of a fourth component on the stability of triple oxides was studied. A stabilizing or destabilizing effect of the deviation from random distribution of rare-earth and alkaline-earth elements over common structural positions was demonstrated.