For the first time solution enthalpies of BaCe0.8Y0.1Gd0.1O2.9, BaCe0.8Y0.1Lu0.1O2.9 and mixtures of BaCl2 + 0.8CeCl3 + 0.1YCl3 + 0.1GdCl3, BaCl2 + 0.8CeCl3 + 0.1YCl3 + 0.1LuCl3 in 1 mol dm−3 HCl with 0.1 mol dm−3 KI have been measured. On the basis of experimental data obtained and literature data the standard molar enthalpies of formation and stabilization energies were calculated. Using Born-Haber cycle the lattice energies were calculated. It was established that lattice energy was increased from BaCe0.8Y0.1Gd0.1O2.9 to BaCe0.8Y0.1Lu0.1O2.9. It correlates with decreasing of ion radius of rare earth element from gadolinium to lutetium. Increasing of lattice energy with decreasing of ion radius of rare earth element was explained with modified formula of Kapustinskii.