Flotation is often adversely affected by uncontrolled oxidation, which is also an electrochemical process. Electrochemical measurements showed that the magnetic 4C type pyrrhotite is anodically more reactive than the non-magnetic 6C type pyrrhotite. It was also shown that the non-magnetic 6C type pyrrhotite is a better substrate for oxygen reduction and is less susceptible to oxidation. When pyrrhotite is in contact with pentlandite, under oxygen-saturated conditions, as the amount of pentlandite increases, the reactivity towards oxidation of the mixed mineral system is reduced. The formation of a continuous oxide layer on the surface and an increase in oxide layer thickness with decreasing pentlandite content were observed. In the presence of xanthate the anodic polarisation diagrams of the pure and mixed samples showed a reduction in the maximum anodic peak current, suggesting the presence of xanthate on the surface, which hinders oxidation of the mineral surface. Electrochemical and ToF-SIMS measurements indicated that CO2 treatment of oxidised pyrrhotite samples resulted in depassivation of the oxidised surfaces.