The influence of adsorption of O₂ and CO on the surface magnetism of a 10 ML film of Fe grown at 120 K on Ag(100) has been investigated by spin polarized metastable deexcitation spectroscopy. The extreme surface sensitivity of this spectroscopy allows us to study the effects of adsorption from exposures as low as ≈ 0.1 L. Oxygen adsorbs dissociatively at 120 K and the O_2p states are spin polarized. At this low temperature, oxidation starts from exposures of the order of 1 L and above 3-4 L a non-magnetic oxide layer is formed. CO adsorbs molecularly at T=120 K and then dissociates above room temperature. Following dissociation, carbon atoms do not have any effect on the spin polarization of oxygen and iron states at the surface.