Photoemission spectra of 3d core levels excited with synchrotron radiation reveal a multicomponent substructure which increases in complexity with oxygen exposures over the range 106–1014L (langmuir). Spectral changes are already evident for Ga at 104 L, and for As near 106 L. Two oxide components shifted by 0.45 and 1 eV relative to the bulk Ga-3d core level are evident throughout the exposure range, but shift to 0.8 and 1.4 eV for 1014 L. With increasing exposure the As-3d core level develops a sequential set of shifted components at 0.8, 2.3, 3.2, and 4.2 eV relative to the bulk position in GaAs, which are attributed to single through fourfold coordinated bond formation to oxygen. Both surface and bulk-sensitive core spectra reveal a nearly equally intense oxide substructure, which indicates that contrary to previous notions subsurface oxidation is the dominant mechanism throughout the exposure range. The core spectra furthermore indicate preferential Ga oxidation—which suggests that separate Ga and As oxide phases form. Thus the oxidation of GaAs(110) is both spatially and chemically inhomogeneous. Changes in the position of the Fermi energy at the surface correlate well with the initial oxidation of surface sites and the onset of subsurface oxidation near 106 L. A final pinning position of the Fermi energy was not observed.