The distribution of rare-earth and strontium cations over two sites in perovskite-like layered aluminates Ln₂SrAl₂O₇, crystallized in the Sr₃Ti₂O₇ structural type, was studied by the full-profile X-ray diffraction analysis. The probability of population of oxygen cubooctahedra (Ln,Sr)O₁₂ with Sr²⁺ cations and of nine-vertex polyhedra (Ln,Sr)O₉ with Ln³⁺ cations was found to increase in the La, Nd, Gd series. This was demonstrated both by direct calculations of the population and by analysis of interatomic interactions within and between layers of the perovskite cells. Cation-oxygen bonds between the layers was shown to strengthen in the above series, which results in stabilization of the layered structure as a whole.