Ru, Ru–Fe and Fe films have been prepared in u.h.v. conditions. In temperature-programmed desorption experiments (t.p.d.) a reasonable desorption rate for both H2 and CO at ca. 550 K (the reaction temperature for the CO + H2 reaction) was observed only for Ru films, especially when the gases were adsorbed at 673 K and cooled to 300 K in H2 and CO, respectively. CO2 formation was observed in t.p.d. only, when large amounts of H2O were present. CH4 was the only product in the CO + H2 reaction at 10 Pa pressure on Ru. On Ru film surface carbon deposited by the dissociation of CH4 or CO could readily be hydrogenated to CH4. A mechanism for the CO + H2 reaction is proposed in which chemisorbed CO having a very weak bond between the carbon and oxygen atoms reacts with hydrogen, resulting in surface carbon and H2O. CH4 forms only in the next hydrogenation step. Iron and ruthenium–iron films, the latter being enriched in iron, proved to be completely inactive in this reaction. A possible explanation is given.