A new method for the determination of thermodynamic properties of metal carbides is described here. The method involves the generation of a metal carbide in situ by heating a mixture of the metal oxide and graphite. The CO pressure in equilibrium with the carbide and the oxide is derived from the rate of effusion of CO from the sintered pellet, as measured by a quadrupole mass spectrometer, and from this data the thermodynamic properties of the carbide are derived. Conditions for obtaining equilibrium pressures are established. This method has been validated by measurements on the Al-C-O system and applied to the Cr-C-O system in order to determine the free energy of formation of Cr7C3 and Cr23C6.