An investigation of important variables which influence ethyl xanthate decomposition by sodium metabisulphite (MBS) during the laboratory flotation of Hilton ore and in plant pulps of the Hilton Concentrator has been undertaken. The decomposition of ethyl xanthate was monitored by Ultraviolet-Visible (W-Vis) spectroscopic examination of the pulp liquid phase. Sulphur speciation in plant pulps was monitored by ion chromatographic (IC) examination of the pulp liquid phase. Simultaneous addition of xanthate and MBS decreased galena recovery due to ethyl xanthate decomposition by sulphite ion derived from MBS. The depression increased with increasing MBS addition and pulp temperature. Under these conditions there was an increase in the concentration of metastable ethyl perxanthate in solution, an intermediate product of xanthate decomposition by sulphite ion. Pulp chemical measurements in the Hilton Concentrator also showed increased ethyl perxanthate concentration when MBS was added at the same point in the flotation circuit as ethyl xanthate. In contrast, the application of an aeration stage after MBS addition, but prior to ethyl xanthate addition, decreased the concentration of unreacted sulphite ion. This was demonstrated by a decreased ethyl perxanthate concentration in solution and restored galena flotation recovery. Under these conditions the dependency of galena flotation on MBS and temperature was less marked. Prevention of the solution decomposition of ethyl xanthate by sulphite allows the depressant action of MBS on sphalerite and iron sulphide flotation to be optimised.