The oxygen nonstoichiometry, δ, and oxidation enthalpy, ΔHox, of double perovskites RBaCo2O6-δ (R = Sm or Eu) were simultaneously measured depending on the temperature and oxygen partial pressure, pO2. Theoretical equations for ΔHox(T, δ) and pO2(T, δ) were derived from the defect structure model based on the oxygen exchange and cobalt disproportionation reactions. These equations were fitted independently to each of the experimental ΔHox(T, δ) and pO2(T, δ) data sets. The resulting enthalpies of defect reactions were found to be almost the same irrespective of the calculation method. In other words, the models, describing satisfactorily the data, can be used to calculate both compositional dependences and redox thermodynamics of RBaCo2O6-δ (R = Sm or Eu). In addition, from the previously published data and the data presented here, trends were determined in the defect reaction thermodynamics of RBaCo2O6-δ (R = La, Pr, Nd, Sm, Eu, Gd, or Y). Drop calorimetric measurements were performed in air to obtain enthalpy increments for RBaCo2O6-δ (R = Sm or Eu) with variable oxygen content because the samples lost oxygen upon being heated in the calorimetric cell. As-obtained data were used to calculate the functional dependences of enthalpy increments of EuBaCo2O5.56 and SmBaCo2O5.6 with a constant oxygen content. In addition, as an example of practical application-oriented calculations for solar energy conversion and oxygen storage, the performances at equilibrium of RBaCo2O6-δ (R = Pr, Sm, Eu, or Gd) were evaluated and compared to those of SrFeO3-δ as a reference material.