A microkinetic model was developed to explain the catalysis of the water-gas shift reaction by sulfided, alumina-supported molybdenum. In this model, the reaction takes place through a regenerative (reduction-oxidation) scheme wherein the catalyst surface is alternately oxidized by water and then reduced by carbon monoxide. The surface of the catalyst is equilibrated with gas-phase H2S under all reaction conditions studied. Coverages predicted by the model are consistent with the adsorption behavior of molybdenum sulfide catalysts. Simulations indicate that the effects of steam and H2S levels are closely related. A maximum in CO conversion with increasing steam level may only occur at certain H2S levels, and the ordering of CO conversion with increasing H2S levels may invert as the steam level is changed.