The heterovalent isomorphism of Ln³⁺ and Ca²⁺ cations in stoichiometric LnCaAlO₄ oxides was studied. The X-ray diffractometry of the intensity of Laue background modulations revealed a positive short-range ordering in Ln³⁺ and Ca²⁺ distribution over points of one structural site common for both ions, which results in formation of stable Ln³⁺-Ca²⁺ couples in adjacent nine-coordinate oxygen polyhedra. The degree of ordering and the interchange energy estimated in the quasichemical approximation depend both on the effective cationic charges and on the number of unpaired f electrons of the lanthanide atom.