Abstract We studied oxygen chemisorption on Ge-covered Si(100) surfaces by AES, XPS and STM. Epitaxial Ge overlayers, 1 and 2 ML, on Si(100) surfaces were oxidized in situ at room temperature. Initial uptake of oxygen adsorption on the surface terminated by a monolayer Ge was reduced by ∼ 1 5 compared with that on a Si(100) surface measured in O KLL AES intensities. The oxygen interaction for the surface covered by 2 ML Ge was in the same range as that of a Ge(100) surface, indicating that the top two layers, surface dimers and the underneath layer, play critical roles in oxygen interaction. The STM study for the surface partly covered by Ge showed that oxygen reacted with the Si substrate in preference to the Ge islands.