Thermochemistry in the decomposition of lanthanum dioxycarbonate, La2O2CO3(s), was studied over the temperature region of 773-1190 K. The decomposition reaction can be written as La2O2CO3(S) = La2O3(S) + CO2(g) The equilibrium property of the above reaction was then studied by tensimetric measurement of the CO2(g) pressure over the biphasic mixture of La2O2CO3(s) and La2O3(S) at different temperatures. The temperature dependence of the equilibrium pressure Of CO2 thus measured could be given by In pCO(2) (Pa)(+/- 0.22) = - 17 502.2/T + 25.87 (773 less than or equal to T (K) less than or equal to 1190) From the above vapor pressure expression, the median enthalpy and entropy of the decomposition of the oxycarbonate from the second law analysis worked out to be 145.5 +/- 5.0 kJ mol(-1) and 119.2 +/- 5.0 J mol(-1) K-1, respectively. The results are discussed in the light of available thermochernical data of the compound.