The interaction of oxygen with a Si(100)–As surface has been studied by a homemade combined surface analysis spectrometry with a molecular beam epitaxy system. This paper confirms that an As layer is a good passivation film for the Si surface and describes a study concerning all processes of oxygen adsorption on a Si(100)–As surface for the first time. It is shown that the oxygen coverage at saturation is 0.5 monolayer, i.e., adsorption sites are reduced by one‐half with the existence of an As layer and the initial sticking coefficientSSi–Asoon the Si(100)–As surface was found to be 5.6×10−3, which is ten times less than that for a clean Si(100) surface. The passivating effect of the As on the Si surface is interpreted with the existence of lone‐pair states instead of dangling‐bond states.