The surface structures formed upon deposition of O-2 and Ga2O onto the technologically important arsenic-rich GaAs(001)-c(2x8)/(2x4) surface have been studied using scanning tunneling microscopy and spectroscopy, and the results are compared to density functional theory calculations. O-2 chemisorbs by displacing first layer arsenic atoms bonded to second layer gallium atoms. Oxygen chemisorption pins the Fermi level at less than 5% monolayer coverage by creating a donor and acceptor site within the band gap originating from the gallium atom bonded between the two O atoms. In contrast, Ga2O chemisorbs by inserting into arsenic dimer pairs at elevated surface temperatures. A monolayer of Ga2O forms a (2x2) surface structure with a crystalline interface that is electronically unpinned: there are no states within the band gap. The unpinned interface results from Ga2O restoring the surface arsenic and gallium atoms to near-bulk charge. (C) 2003 American Institute of Physics.