Room temperature adsorption of oxygen on the Si(111) 2×1 surface and the effects of annealing of oxygen-covered surfaces were studied with photoemission techniques. Before annealing, the dominant chemical shifts in Si-2p are ∠0.9 and ∠1.4 eV; after annealing, a dominant shift at ∠2 eV is found. Possible chemisorption configurations are discussed in terms of the new information gained and many possibilities are eliminated. However, a few Si–O bonding configurations remain; more precise calculation of chemical shifts will be necessary to choose between them.