The one-, bi- and tri-component (K)(Ni)(Mo)gamma-Al2O3 systems are studied after water-gas shift reaction tests in the presence of sulfur in the feed. The samples are characterized after the calcination procedure (oxidic state) and after the catalytic activity test (tested state) by SSA, XRD, XPS and EPR measurements. The Mo4+, Mo5+ and Mo6+ oxidation states are registered on the surface of the tested samples. The different sulfur species such as: sulfide sulfur, oxysulfides, polysulfides, paramagnetic sulfur and sulfate sulfur are present on the sample surface in the reaction redox environment (CO, H2O, H2S, H-2 and CO2 agents). The XPS and EPR characterization of the tested state points to the idea that both the molybdenum reducibility (i.e. the Mo4+/(MO5+ + MO6+) and MO4+/MO5+ ratio.;) and the sulfur lability (sulfide/sulfate ratio) represent informative indices in regard to the catalytic properties of the Mo-containing systems in the WGS reaction in case of sulfur presence in the feed mixture. (c) 2005 Elsevier B.V. All rights reserved.