Although TiN has hitherto been the most extensively investigated hard coating, other binary and ternary nitrides are the subject of ever-increasing interest. In this work the behaviour of TiN. ZrN and CrN coatings is compared. concentrating on the mechanism of their oxidation. The sequence of the processes included in the electrochemical and thermal oxidation was followed by X-ray photoelectron spectroscopy (XPS). It is shown that the behaviour of TiN and ZrN coatings is qualitatively similar. whereas CrN exhibits a quite different behaviour. Valence band spectra of these nitrides and their related oxides are presented and discussed.