While the oxidation stages of silicon beyond one monolayer have been extensively documented by photoemission and Auger experiments, the question as to whether initial chemisorption of oxygen is molecular or dissociative is controversial. We have carried out theoretical calculations using the extended tight‐binding method simulating both O2and O overlayer adsorption on Si(111) surface. Comparison of published and our new uv photoemission spectra with our calculated density of states indicates a better agreement with the dissociated oxygen model than with the molecular one. We have also studied the x‐ray photoemission of oxygen on Si(111) from submonolayer to several monolayers of oxide (∠ 8Å). Our data is consistent with initial dissociative adsorption but incompatible with molecular adsorption.