Investigations concerning the magnetic interactions of oxygen with thin cobalt films evaporated on W(110) were carried out by spin resolving photoelectron spectroscopy. In particular, modifications of surface magnetism depending on the chemical state were determined. The formation of different oxidation states at various substrate temperatures could be demonstrated. At room temperature a coexistence of chemisorbed oxygen and antiferromagnetic CoO near the surface was found. With increasing exposure a reduction of the exchange splittings of the oxygen derived bands can be observed for different excitation energies. At low temperatures the formation to Co 3O 4 starts immediately after dosing oxygen to the cobalt surface.