The vapour species over Al2O3(c) were determined mass spectrometrically from an alumina effusion cell. These data are in good agreement with previously published weight loss data since the partial pressures of the suboxides, AlO, Al2O, AlO2 and Al2O2 were found to be of the same order as those of the elements, Al and O. New values were determined for the ΔHf298K of AlO, Al2O, AlO2 and Al2O2 of 17.0 ± 2.0, –33.2 ± 3.0, –44.3 ± 2.0 and –113.6 ± 2.0 kcal/mol, respectively. Excellent agreement was obtained between second and third law studies.