The interfacial chemistry and geometry of Au/oxidized GaAs has been investigated using monochromatic X-ray photoelectron spectroscopy (XPS). Both Au-deposition and interface characterization were performed in situ in an ultra-high vacuum XPS chamber. At the initial stage of Au deposition, the pre-formed surface bonding state of As-O prior to Au deposition was completely changed into thermodynamically more stable Ga-O bonding with the kinetic and condensation energies released from Au deposition. The irradiated Au atoms on the GaAs surface produced a mixed state with GaAs. The composition of this Ga-As-Au mixed phase was initially excess-As GaAs due to the liberated As through the bonding transition of oxides. With the successive deposition of Au it gradually changed to excess-Ga state because of the high volatility of As. In particular, this mixed phase was revealed to locate between Ga oxides and Au overlayer. Based on the layered structure model, the quantitative simulation on the Au/GaAs interface could elucidate the resultant bonding-layered structure of interface; Au/Ga-As-Au mixed layer/Ga oxides/GaAs.