The interaction of NO 2 on clean reconstructed Si(100) surfaces has been studied by X-ray photoelectron spectroscopy (XPS) in large substrate temperature ( T S) and NO 2 pressure ranges. Our results support the adsorption of only completely dissociated or atomic species whatever T S between room temperature and 1100 K. The respective oxygen and nitrogen uptakes, given as a function of pressure and T S, are compared. For oxygen, a critical temperature T O c (pressure P O c) for a given pressure P O c (temperature T O c) separates the desorption or combustion mode from the passivating one, very similar to the oxidation by O 2. For nitrogen, similar evolutions with T S and pressure are evidenced only above 750 K, when oxygen desorption occurs. Below 750 K, oxidation prevails over nitrogen chemisorption with a nitrogen coverage limited to a fraction of a monolayer whatever the pressure. Above 750 K the nitrogen chemisorption rate is enhanced so that the T S range comprised between 750 K and T O c defines an oxynitridation stage where both substantial oxygen and nitrogen coverages are found. Above T O c, nearly pure nitridation is enabled by oxygen etching. In conjunction with coverage measurements, angle-resolved XPS was used to probe the in-depth distribution of the chemisorbed atoms in the T S range above 750 K. From the comparison of the O 1s and N 1s intensities it is concluded that the initial oxynitride occurs with the formation of nitrogen pile-up at the Si/Si-oxide interface.