The effect of sulphite ions on the xanthate-induced flotation of copper-activated pyrite has been studied. Various techniques have been used to identify the plausible mechanisms of interaction of sulphite with both collector and pyrite surface. It was found that sulphite depressed the xanthate- induced flotation of copper-activated pyrite with nitrogen or air purging at pH 7. However, the depression effect was greater when air was used. Solution and spectroscopic studies indicated that sodium bisulphite interacted with the pyrite surface, as well as with isobutyl xanthate in solution and its adsorbed state. Based on the results obtained, a combination of possible mechanisms has been proposed to explain the depressing effect of sulphite on copper-activated pyrite. It was suggested that both the decomposition of the xanthate in solution via perxanthate formation and dixanthogen desorption from the pyrite surface were responsible for the pyrite depression. Furthermore, iron hydroxy species were formed on the pyrite surface, rendering it hydrophilic, thus preventing the formation of bubble-pyrite particle aggregates.