CO partial pressures over the univariant three-phase region containing NdO 1 . 5 (s), C(s) and NdC 2 (s) were measured by using a quadrupole mass spectrometer (QMS) between 1403 and 1588 K. The NdC 2 (s) phase was generated in situ starting from a mixture of NdO 1 . 5 (s) and C(s). The second-law and third-law enthalpies of the reaction NdO 1 . 5 (s)+3.5C(s)=NdC 2 (s)+1.5CO(g) at 298 K were calculated. The enthalpy of formation of NdC 2 (s) was derived from the enthalpy of reaction and the enthalpies of formation of NdO 1 . 5 (s) and CO(g) taken from the literature. The Gibbs energy of formation of NdC 2 (s) was derived from the Gibbs energy of formation of NdO 1 . 5 (s) and CO(g) from the literature. The third-law enthalpy of the reaction is (648.1+/-1.0) and (647.6+/-1.0)kJmol - 1 based on thermal functions of NdC 2 (s) derived from those of UC 1 . 9 4 and ThC 1 . 9 4 , respectively. The recommended enthalpy and Gibbs energy of formation of NdC 2 (s) at 298 K are -(90.8+/-6.0) and -(122.2+/-6.0)kJmol - 1 , respectively.