Separation of pyrrhotite from pentlandite is highly important in flotation practice of massive nickel sulphide ores. Flotation reactivity of these minerals was studied using cyclic voltammetry and rest potential measurements, which included dissolved oxygen concentration and butyl xanthate addition as the main parameters at a solution pH of 9.2. Under the experimental conditions used, the voltammograms, obtained in the absence and presence of xanthate (at 0.1 mol m(-3)) and dissolved oxygen, provided no direct evidence for any anodic peaks that can be associated with xanthate electroadsorption or its possible transformation to dixanthogen, as an agent of superior hydrophobicity. However, the current densities corresponding to the most prominent anodic potential for pyrrhotite were noticeably lower in the presence of xanthate, suggesting its involvement in the formation of a partially passivating film. According to results of rest potential measurements, formation of dixanthogen will be common to both of these minerals in oxygen-saturated solutions to promote their bulk flotation. However, under oxygen-deficient conditions in the presence of xanthate, pyrrhotite develops rest potentials that are much lower than the equilibrium potential for dixanthogen formation. Since dixanthogen formation is essential for pyrrhotite floatability, controlling the dissolved oxygen level and potential in the pentlandite-pyrrhotite flotation environment at low levels is expected to promote selective flotation of pentlandite, which has been demonstrated independently.