The paper presents the results of studying the catalytic properties of “metal-carbon” systems using the method of surface ionization of CsCl molecules. Using the Ir-C system as an example, it was shown that the poisoning effect of the same poison (carbon) strongly depends on the structure of the layer formed on the catalyst. In metals that do not form bulk carbides, the carbon monolayer completely poisons the catalytic properties of these metals. It is shown that individual atoms and clusters of samarium deposited on iridium coated with a monolayer of graphite lead to efficient catalytic dissociation of CsCl molecules. The mechanism of formation of a carbon film on the surface of a metal obtained by diffusion from the volume, which differs from the mechanism of formation of a carbon film obtained on the surface of a metal by adsorption, has been elucidated.