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 ln pCO₂ (Pa)(±0.22) = - 17 502.2 / T + 25.87 (773 ≤ T (K) ≤ 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 ± 5.0kJ mol^-1 and 119.2 ± 5.0J mol^-1 K^-1 , respectively. The results are discussed in the light of available thermochemical data of the compound.