Thermochemistry in the decomposition of samarium di-oxycarbonate, Sm2O2CO3(s) and europium di-oxycarbonate Eu2O2CO3(s) was studied over the temperature regions of 755-987 K and 773-989 K, respectively. The equilibrium properties of the decomposition reactions were obtained by tensimetric measurement of the CO2(g) pressure over the biphasic mixture of RE2O2CO3(s) and RE2O3(s) at different temperatures (RE= Sm, Eu) and also by thermogravimetric analysis of the decomposition temperature at 1 atmosphere of CO2 pressure. The temperature dependence of the equilibrium pressure of CO2 thus measured could be given by In p(CO2) (Pa) (+/-0.13)= -15722.8/T + 26.83 (755 <= T(K) <= 987) for Sm2O2CO3 decomposition and In p(CO2) (Pa) (+/-0.13)= -18615.7/T + 29.90 (773 <= T(K) <= 989) for Eu2O2CO3 decomposition. From the above vapor pressure expressions, the median enthalpy and entropy of the decomposition of the oxycarbonates were calculated by the second law analysis and their thermodynamic stabilities were derived. The results are discussed in the light of available thermochemical data of the compounds. (C) 2008 Elsevier B.V. All rights reserved.