The electrical properties of MOS devices are critically dependent on the oxide-semiconductor interface. The preparation of suitable insulating layers of oxide or other material is essential for the performance of such devices and it is particularly difficult in the case of III-V compound semiconductors. We report a method of preparing an insulating layer on InSb by a plasma oxidation process. The oxidation method will be described as well as results of the analysis of the oxide-semiconductor interface by electrical and compositional techniques. Capacitance-voltage characteristics reveal the existence of interface states which are distributed near the conduction and the valence bands with a higher density near the former. Depth profiling of the oxide by Ar+ sputtering and Auger electron spectroscopy (AES) shows that the oxide is composed of a mixture of indium oxide with the antimony oxide. At the interface a transition region can clearly be observed, whose composition seems to be mostly indium oxide rich in metallic antimony. This information can be derived from the Auger line shapes and positions. We find AES to be an extremely helpful tool for this kind of investigation, in spite of the obvious disadvantages of ion sputtering, especially in the case of indium and antimony. Correlations of the AES data with the electrical analysis results are suggested.