High temperature oxide melt solution calorimetry using molten 2PbO·B 2 O 3 at 1078K and molten 3Na 2 O·4MoO 3 at 976K has been applied to determine standard enthalpies of formation, Δ f H m ° (298K), of Ln 2 O 2 CO 3 II (Ln=La, Nd, Eu) from their oxide components Ln 2 O 3 and CO 2 at 298K. Two solvents were applied to cross-check the thermodynamic cycles. Using molten 2PbO·B 2 O 3 , Δ f H m °(298K) for La 2 O 2 CO 3 II, Nd 2 O 2 CO 3 II and Eu 2 O 2 CO 3 II were found as −198.0±1.7, −176.2±3.1 and −151.4±6.5kJmol −1 , respectively. Using molten 3Na 2 O·4MoO 3 the corresponding values for Nd 2 O 2 CO 3 II and Eu 2 O 2 CO 3 II were determined as −176.6±3.5 and −154.9±3.2kJmol −1 , respectively. These enthalpies of formation are discussed in view of prior thermodynamic data, discrepancies are analyzed, and recommended values for enthalpy, entropy and free energy of formation for La 2 O 2 CO 3 and Nd 2 O 2 CO 3 are given.