We have investigated the effect of chemisorption on the magnetic properties of metastable layers of fcc Co(001) and fcc Fe(001) epitaxed to Cu(001) substrates. The Curie temperature has been observed to fall rapidly with decreasing film thickness below 10 monolayers,1 so that the low-temperature chemisorptive properties can be studied. We have observed the binding energies of various adsorbates using thermal desorption kinetics as a function of magnetic film thickness. The magnetic behavior of the films was monitored in situ using the surface magneto-optic Kerr effect (SMOKE) method,2 and changes in the electronic states at the Fermi level were monitored with high-resolution photoelectron spectroscopy. The question of the basis for a so-called "magnetocatalytic effect"3 is addressed in the light of these results.