We have prepared polycrystalline Co films on epitaxially oxidized Si(111) surfaces and studied their oxidation kinetics under atmospheric conditions using the fact that metallic cobalt is a ferromagnet but CoO is an antiferromagnet and therefore practically nonmagnetic. As a consequence, oxidation is associated with a loss in magnetism. Results show that all samples with an initial thickness of di ≳ 5 nm oxidize practically instantaneously, whereby a constant amount of 2.5 nm of metal is transformed into oxide. For di < 5 nm the time constant for oxidation increases considerably and follows an approximately linear dependence with decreasing film thickness, reaching an extrapolated value of τ=190 days for di → 0. This increasing time constant let all samples with di < 2.5 nm appear ferromagnetic within the course of this study due to a nonoxidized metallic rest. Auger electron spectroscopy analyses revealed that the main oxidation product is in fact CoO, especially near to the metal interface, but that other compounds are also likely to be formed summing up to a total thickness of 6 nm including adsorbates. A second point of interest was the unidirectional magnetic anisotropy which the oxide imposes to the metal due to an exchange interaction at the interface. This anisotropy shows a sharp onset at a CoO thickness as small as ∼0.25 nm from which an anisotropy constant of ∼4.5×106 J/m3 can be estimated for ultrathin CoO layers.