The paper summarizes experimental and theoretical evidence of an oxidative addition of hydrogen to the active metal (Ni or Co) in the active component of the sulfide HDS catalysts-a new phenomenon unknown to date in heterogeneous catalysis. Several aspects are discussed: thermodynamics of the oxidative addition of hydrogen; possible mechanism of dihydrogen activation by sulfide catalysts; electronic state of the active metals Ni(IV) and Co(III) with d(6)-electron configuration; reversibility of the oxidative addition and reductive elimination of hydrogen at high temperature; sites of hydrogen localization inside the active component matrix; spectroscopic evidence on the existence of "occluded" hydrogen; a strong interaction of the occluded hydrogen with the active component matrix; a new version of the catalytic cycle of thiophene hydrogenolysis. (C) 2002 Elsevier Science B.V. All rights reserved.