Molybdenite (MoS2) concentrates are produced by differential flotation from bulk copper-molybdenum concentrates. However, complete separation of molybdenum by flotation is very difficult, thus, molybdenum concentrates produced by this route requires a further step of chemical purification in order to reduce the copper content to a suitable level for marketing. Refractory chalcopyrite (CuFeS2) is the major copper mineral impurity in these concentrates; thus the elimination of copper by leaching requires highly aggressive solutions. This paper is concerned with a novel process of chemical purification of molybdenite concentrates containing chalcopyrite as the main copper impurity. The process involves a sulfidation of the molybdenum concentrate with S2(g) at 380°C followed by a leaching stage. H2SO4-NaCl-O2 leaching of a sulfidized concentrate containing 3.4% Cu for 90 min at 100°C produced a molybdenite concentrate with less than 0.2% Cu, which is appropriate for marketing. Molybdenite dissolution was negligible in these conditions.