Two methods of phase analysis for solids with complex compositions have been developed in our previous investigations. The first method is thermodynamic analysis which enables a phase diagram to be plotted and by means of this the spatial distribution of phases as a result of chemical interactions between two different solids at an interface may be predicted. The second method is cyclic voltammetry using carbon paste electrodes; with this method it is possible to analyse the phase composition of small objects, e.g. thin (about 0.01 μm) films. The limit of detection of phases by this method is about 10 15 cm -3, i.e. about two orders of magnitude lower than that of modern expensive Auger electron spectroscopy, electron spectroscopy for chemical analysis and other surface analysis methods. Both methods were applied to the InSb/native oxide system. Using the first method three alternative ternary In-Sb-O diagrams were constructed and the experimental data obtained with the second method permitted us to select the most probable diagram. Our results showed that thin (about 0.1 μm) native oxide films on InSb have multilayer structures in which each layer has a different phase composition. When the oxidation occurs under quasi-equilibrium conditions the phase compositions from the InSb surface to the top of the film are as follows: I, In 2O 3+Sb; II, In 2O 3+Sb 2O 3+Sb; III, In 2O 3+Sb 2O 4; IV, InSbO 4. However, when the In-Sb-O system is far from equilibrium during oxidation, unstable phase complexes such as Sb 6O 13+Sb 2O 3 or In 2O 3+Sb 2O 5 may be observed; films obtained by anodic oxidation of InSb in 0.1 M KOH are an example of this situation.