For the purpose of determining the thermodynamic stability of gamma-aluminum oxynitride spinel, alon, equilibrium N-2-CO gas compositions coexisting with alon-Al2O3-graphite have been measured by means of gas chromatography and quadrupole mass spectrometry in the temperature range from 1908 to 2023 K. From the results obtained, the standard Gibbs energy change of the carbothermal nitridation reaction of Al2O3 to form alon 64 + x / 3x Al2O3(s) + 3C(s) + N-2(g) = 2 / x Al(64+x)/3V (8-x)/3O32-xNx(Al2O sat.) + 3CO(g) has been determined to be Delta(r)G(o)/kJ = 726.2 - 0.3611 T (+/-0.7) (1908-2023 K) where alon is expressed as Al(64+x)/3V(8-x)/3O32-xNx (V is vacancy). Using the obtained Gibbs energy change, the eutectoid decomposition temperature of alon into alpha-Al2O3 and AlN has been evaluated to be 1903 +/- 4 K, and the chemical potential diagram of the Al-O-N-C system has been constructed at 2000 K. (C) 2003 The Electrochemical Society.