The results of a comparative study of the resistance to oxidation of AIN and a codeposit of AlN-Si3N4 a re presented. The oxidation of both types of layer was performed at 1200 degrees C in an oxygen gas flow (p(O2) similar to 1 atm). A thermogravimetric analysis was made, recording the weight gain throughout the oxidation process, and revealed a dramatic improvement in the resistance to oxidation of the codeposit layer when com pared with the aluminium nitride layer. An X-ray photoelectron spectroscopy depth-profile analysis was made by performing a series of successive abrasive polishes on the oxidized layers. After each abrasive polish the sample surface was cleaned by ionic bombardment under ultrahigh vacuum and the photoemission spectra recorded. This process of polishing, etching and analysis was continued until the oxygen content decreased to a level comparable with that observed in the layers before oxidation. The changes in chemical bonding throughout the various depth profiles were examined by deconvolution of the Al 2p and Si 2p photoelectron peaks and by analysis of the Auger KLL lines. It was concluded that the slower rate of oxidation of the codeposit could be attributed to either the formation of silica or a mixed oxide phase of the mullite type (3 Al2O3-2 SiO2).