The oxidation of Si0.5 Ge0.5 alloy has been investigated over the temperature range 800 to 1000°C . The composition of the oxide layers has been determined by 1.5 MeV Rutherford backscattering spectroscopy (RBS), infrared transmission spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS). During the initial stage of oxidation, as the temperature ramps up in an oxidising ambient both Si and Ge are oxidized to form a mixed oxide in the region near the surface. Upon further oxidation, at temperatures higher than 900°C and times longer than 5 minutes, the Ge atoms are ejected by the growing oxide layer, which has the composition of Si02, and accumulate in the underlying alloy, which becomes rich in Ge.The surface oxide layer remains unchanged. The proportion of Ge02 in the near surface region, determined from IR spectra, decreases when the temperatures increases from 800°C to 1000°C. It is concluded that at 800°C Ge atoms are insufficiently mobile to be ejected from the growing oxide and both matrix species ( Si and Ge ) have equal probabilities of oxidation leading to the formation an oxide of composition Si0.5Ge0.5O2 . Supporting experiments confirm that Ge02 in the surface layer is formed during warm up of the samples oxidized at 900 and 1000°C.