The surfaces of TiO2 and ZnO nanoparticles have been modified by gold (Au) nanoparticles by a reduction method in solution. Their interfacial electronic structures and optical absorptions have been studied by depth-profiling X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy, respectively. Upon Au-modification, UVvis absorption spectra reveal a broad surface plasmon peak at around 500 nm. For the as-prepared Au-modified TiO2 and ZnO, the Au 4f7/2 XPS peaks exhibit at 83.7 and 83.9 eV, respectively. These are due to a charge transfer effect from the metal oxide support to the Au. For TiO2, the larger binding energy shift from that (84.0 eV) of bulk Au could indicate that Au-modification site of TiO2 is different from that of ZnO. On the basis of the XPS data with sputtering depth, we conclude that cationic (1+ and 3+) Au species, plausibly Au(OH)x (x = 1-3), commonly form mainly at the AuTiO2 and Au-ZnO interfaces. With Ar + ion sputtering, the oxidation state of Ti dramatically changes from 4+ to 3+ and 2+ while that (2+) of Zn shows no discernible change based on the binding energy position and the full-width at half maximum (FWHM).