We use scanning tunneling microscopy, atom-resolved tunneling spectroscopy, and electronic structure calculations to determine the nature of the adsorption state of oxygen in the initial stages of the oxidation of Si(111). We are able to directly image two states of adsorbed oxygen. One of them is identified as a Si adatom site with one oxygen atom inserted in one of the back bonds, while the other involves an oxygen atom tying up the adatom dangling bond with, most likely, another oxygen inserted in one of the back bonds. As the coverage is increased toward the monolayer, the latter site becomes the dominant one. © 1990 American Chemical Society.