The presence of copper(II) ions in the process water used in the mineral processing industry has an impact on iron and nickel sulphide mineral flotation in both acidic and alkaline environments. Millerite (β-NiS) – a nickel sulphide mineral, has been found misreported to Cu concentrate, which negatively impact the performance of Cu/Ni separation. In this paper, we present a fundamental study on the effect of Cu(II) ions on millerite flotation and surface properties in alkaline solutions. Micro-flotation tests indicated that millerite flotation can be activated by conditioning with copper ions prior to KEX addition at both pH 9 and 12. The surface chemistry of millerite in the presence of Cu(II) was studied by mineral dissolution and EDTA extraction tests as well as XPS analysis to probe the underlying activation mechanism. At alkaline pH, the main copper species on millerite surface was Cu(I) sulphide with minor Cu(II) sulphide and Cu(II) oxide/hydroxide. The Cu(I) sulphide arose from the adsorption and reduction of Cu(II) species. Meanwhile, sulphur oxidation was affected upon the reduction of Cu(II) to Cu(I). Monosulphide species (S2-) at the surface was oxidized into disulphide species (S22-), resulting in less sulphoxy species (SxOy2-). There was no remarkable increase in nickel dissolution and growth in nickel hydroxide passivation layer upon the adsorption of Cu. In essence, copper ion activated millerite flotation under alkaline pH through affecting millerite surface oxidation.