On the basis of the fundamental concepts of catalysis by transition metal sulfides (TMSs) a new dynamic model of the promoted Mo-sulfide catalysts functioning under HDS conditions has been developed. According to the model under hydrogen atmosphere sulfidation degree of the edges of different layers of the slab can vary and surface sulfur can migrate from one layer to a neighboring layer so that average sulfidation state of the slab remains constant under given reaction conditions. file model describes oscillations by sulfur and promoter atoms between the adjacent layers of the slab on the atomic scale level. The driving force of such processes is the heterolytic dissociation of the gas phase hydrogen and formation of hydride hydrogen linked to a promoter atom. This hydride hydrogen triggers an electron transfer front a promoter to Mo with the formation of the vacancy. This model correlates with radioisotopic data and an earlier Suggested "forcing out" mechanism and gives some new interpretations of the important statements of the electron theory of promotion and the "rim-edge" model. (C) 2009 Elsevier B V All rights reserved.