The effect of sulphite on the collectorless flotation of chalcopyrite has been studied, while the mechanism of interaction between sulphite and chalcopyrite surfaces has been investigated by X-ray photoelectron spectroscopic (XPS) and dissolution kinetic studies. The effect of sulphite on the collectorless flotation of chalcopyrite strongly depends on the state of the chalcopyrite surface prior to sulphite conditioning. In the case of a chalcopyrite surface which was only moderately oxidised, exhibiting significant collectorless flotation, sulphite strongly depressed its flotation. This was attributed to the decomposition of a sulphur-rich surface phase by adsorbed sulphite. However, in the case of chalcopyrite which had significant surface concentration of adsorbed iron oxyhydroxide, for which the collectorless flotation was low, sulphite increased chalcopyrite collectorless flotation. This was attributed to the removal of the interfering iron oxyhydroxide and concomitant exposure of the sulphur-rich sub-layer. A mechanism involving reduction of adsorbed ferric oxyhydroxide to more soluble ferrous species is proposed to explain this behaviour.