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This paper describes a concerted mechanism for the hydrogenolysis of the carbon-heteroatom bond of thiophene, pyrrole and furan (and homologs) on mol bdenum or tungsten sulfided catalysts (promoted by Co, Ni or Fe). The five-membered rings would adsorg by a bond on a coordinatively unsaturated Mo (or W), with their heteroatom (S, N or 0) interacting, as electron acceptors, with the sulfur atoms of two vicinal sulfhydryl groups. After a 2,3 semi-hydrogenation, a concerted mechanism would bind the hydrogen atoms of the sulfhydryl groups to the carbon atoms in the 1 and 4 positions, while the hydrogen atoms initially attached to these 1 and 4 carbon atoms would form H2S with the thiophenic sulfur (or NH3 or H20). The mechanism explains most published features of the reactions and explains the special selectivity of sulfide catalysts, especially with furan.