Seawater flotation has been applied to mineral processing in areas located far from fresh water resources. However, as seawater has a detrimental effect on molybdenite floatability under alkaline conditions (pH > 9.5), its application in the conventional copper and molybdenum (Cu-Mo) flotation circuit is hindered. A fundamental study of the effect of two divalent cations in seawater, Mg2+ and Ca2+, on the floatability of chalcopyrite and molybdenite is presented in this paper. Floatability tests showed that both MgCl2 and CaCl2 solutions depress the floatability of chalcopyrite and molybdenite at pH values higher than 9. Furthermore, Mg2+ exerts a stronger effect than Ca2+ owing to the adsorption of Mg(OH)(2) precipitates on the mineral surfaces, as indicated by dynamic force microscopy images. The floatability of chalcopyrite was significantly depressed compared with that of molybdenite in a 10(-2) M MgCl2 aqueous solution at pH 11. This phenomenon is likely due to the adsorption of hydrophilic complexes on the mineral surface, which reduces the surface hydrophobicity. A reversal of the zeta potential of chalcopyrite in MgCl2 and CaCl2 solutions at pH 11 and 8, respectively, indicated the adsorption of precipitates onto the surface. In contrast, the zeta potential of molybdenite decreased" continuously under the same conditions. The floatability test of chalcopyrite and molybdenite in mixed systems showed that selective separation of both minerals should be possible with the addition of emulsified kerosene to a 10(-2) M MgCl2 solution at pH 11. A mechanism is proposed to explain this phenomenon. (C) 2016 Elsevier Ltd. All rights reserved.