The interaction of oxygen with thin cobalt films supported on oxidized Si(100) substrates was studied using Auger electron spectroscopy (AES) and work function changes (Delta phi) at 130 and 300 K. Oxygen uptake curves were derived from the ratio changes of the O-KLL and Co-LMM Anger signals at 513 and 778 eV, respectively. They showed a constant sticking coefficient (s(o)) up to approximate to 15 langmuirs, especially at 130 K, possibly indicating adsorption through a molecular precursor that diffuses over the chemisorbed layer. At 300 K, s(o) = 0.2, while at 130 K, 0.5 greater than or equal to s(o) greater than or equal to 0.3, depending on the dosing pressure. Up to approximate to 10 langmuirs O-2 at 300 K, the work function increased 0.20 eV. At that coverage, the Co MVV Auger transitions indicated an oxide formation. From this coverage onward, the work function decreased, saturating at approximate to 80 langmuirs with a value approximate to-1.2 eV below the clean Co surface, pointing to oxygen diffusion into the bulk.