The activation of sulphide minerals by heavy metal ions in acidic pH solutions has been reviewed from an electrochemical point of view. It is shown that the well-documented ion-exchange model is oversimplified. Surface oxidation of the mineral and reduction of the activator metal ions occur during the activation processes. Activation in an oxygen-deficient system leads to the formation of a sulphur-rich surface and results in activation-induced flotation. The overall process of activation is controlled by a mixed-potential mechanism.