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The spent catalysts discarded from the petroleum refinery operations contain a substantial amount of valued metals such as Ni, Mo and V together with high sulfur content. Recently, the smelting reduction process of this resource using the carbothermic reaction is being developed and commercialized to recover these metals in the form of carbon saturated ferro-alloys containing Ni, Mo and V. Sulfur can be picked up into alloy melts from the sulfur bearing spent catalysts during the smelting process. Therefore, thermodynamics of sulfur in these alloy melts is very important for producing low sulfur ferro-alloys. In the present study, thermodynamic interactions between those alloying elements and sulfur in liquid iron was studied using the slag/metal equilibration technique at 1 873 K. The equilibrium sulfur distribution was measured between a slag with a known sulfide capacity and carbon saturated liquid Fe-V-Ni-Mo alloys of various compositions. The carbon solubility in liquid alloy was significantly changed with alloy composition. The specific effects of V, Ni and Mo on sulfur was determined by considering the effect of carbon on sulfur using Wagner's formalism as the first-and second-order interaction parameters as well as the second-order cross-product terms.