A study is made on the reactivity of a molybdenite concentrate (46.7% Mo) in response to chemical attack, using HNO3, H2SO4 of mixtures or both acids, or to attack by bacteria (mesophiles and moderate or extreme thermophiles). Leaching with sulphuric acid does not attack either of the metallic compounds of the concentrate, while leaching with nitric acid or with mixtures of both acids may globally attack the concentrate or selectively dissolve the chalcopyrite presents in the concentrate. Mesophilic and moderate thermophilic microorganisms attack the chalcopyrite only partially; nevertheless, the use of extreme thermophilic bacteria allows the chalcopyrite to be dissolved selectively, giving rise to a practically pure molybdenite concentrate.