The interaction of oxygen with a carburized Mo(100) surface was investigated at different temperatures (300-1000 K). The different information depths of low-energy ion-scattering (LEIS) spectroscopy, with topmost layer sensitivity, Auger electron spectroscopy (AES), and angle-resolved X-ray photoelectron spectroscopy (ARXPS) allowed us to discriminate between reactions on the topmost layer and subsurface transformations. According to ARXPS measurements, a carbide overlayer was prepared by the high-temperature decomposition Of C2H4 on Mo(100), and the carbon distribution proved to be homogeneous with a MO2C stoichiometry down to the information depth of XPS. O-2 adsorbs dissociatively on the carbide layer at room temperature. One part of the chemisorbed oxygen is bound to both C and Mo sites, indicated by LEIS. Another fraction of oxygen atoms probably resides in the hollow sites not occupied by C. The removal of C from the outermost layer by 02, in the form of CO, detected by mass spectroscopy (MS), was observed at 500-600 K. The carbon-depleted first layer is able to adsorb more oxygen compared to the Mo2C/Mo(100) surface. Applying higher doses Of 02 at 800 K results in the inward diffusion of 0 and the partial oxidation of Mo atoms. This process, however, is not accompanied by the removal of C from subsurface sites. The depletion of C from the bulk starts only at 900 K (as shown by MS, AES, and XPS), very probably by the diffusion of C to the surface followed by its reaction with oxygen. At T-ads = 1000 K, the carbon content of the sample, down to the information depth of XPS, decreased further, accompanied by the attenuation of the C concentration gradient and a substantially decreased amount of oxygen.