A critical analysis of vapor pressure determinations over the ternary Al-C-O system, i.e., mainly the pseudobinary section Al2O3-Al4C3, is performed in order to further explain the behavior of alumina in the presence of carbon (in contact or not) in vacuum furnace environments. The vapor pressure studies allow the determination of standard enthalpies of formation of the oxycarbides, mainly for Al4CO4:Delta H-f degrees(Al4CO4,s,298.15 K) = - 2303 +/- 24 kJ mol(-1). Values as retained are compared with those used in phase diagram and thermodynamic data optimizations. The determination of thermodynamic data for the other oxycarbide Al2CO is not possible in the context of vapor pressure measurements because nonequilibrium vaporization occurs in concordance with metastable phases. (c) 2006 The Electrochemical Society.