Heat capacities and enthalpies of formation of BaGd2O4 were determined by high-temperature differential scanning calorimetry and high-temperature oxide melt solution calorimetry, respectively. Thermodynamic stability of BaLn(2)O(4) compounds increases with decreasing Ln(3+) ionic radius. Previously reported data on BaNd2O4 and BaSm2O4 corroborate this trend. Missing data for compounds in BaO-Ln(2)O(3) (Ln = La, Pr, Eu, Er) systems were estimated from established relations, thermodynamic assessment was performed, and binary phase diagrams were calculated.