Surface oxidation has a significant influence on sulfides flotation. However, there is a little literature about the effect of surface oxidation in the flotation of mixed copper-nickel sulfide (Cu-Ni sulfide, mainly composed of pentlandite, chalcopyrite, pyrite and pyrrhotite). In this work, the effect of surface oxidation on the Cu-Ni sulfide flotation was studied. Flotation results showed the surface oxidation had a stronger depression effect on Cu-Ni sulfide flotation than that on the flotation of single pyrite and chalcopyrite mineral samples. XPS analysis results showed that the hydrophilic products sulfate and Fe/Ni oxide/hydroxide were generated on the oxidized Cu-Ni sulfide surface, whereas no Cu oxide/hydroxide was found, indicating the occurrence of galvanic interaction, which might cause the stronger depression of oxidation on Cu-Ni sulfide flotation. However, after the pretreatment of ultrasonic cleaning in HCl solution, the sulfate and iron hydroxide/oxide on Cu-Ni sulfide surface decreased significantly, and then Cu-Ni sulfide floatability was regained to some extent. Interestingly, adsorption results exhibited that the adsorption amounts of PBX on Cu-Ni sulfide surface basically unchanged before and after oxidation, which might be caused by that chalcopyrite and pyrite were protected from oxidation as the cathode minerals in the galvanic interaction. (C) 2020 Elsevier B.V. All rights reserved.