A short-range ordering of lanthanide and calcium atoms was found in isomorphous La³⁺ and Ca²⁺ cations in stoichiometric oxides LnCaAl₃O₇ (Ln = La, Nd, Sm, Gd, Ho, Er). It takes place almost exclusively within the first cation coordination sphere. The cations form electrostatically linked mixed cation pairs Ln³⁺-Ca ²⁺ in the nearest neighboring points of a common structural site. No spatial ordering of the pairs themselves is observed.