Surface analysis results by X-Ray Photoelectron Spectroscopy (XPS) of samples taken from the Hilton Concentrator of Mount Isa Mines Ltd are presented. Flotation rate data taken during the same sampling period are also presented and discussed. Pulp chemical measurements during the sampling period included Eh, pH, dissolved oxygen, temperature, dissolved metal ion analysis and metal ions extractable by ethylenediaminetetra acetic acid (EDTA). The focus of this paper is on the galena flotation section of the Hilton Concentrator Historically the galena exhibits a low flotation rate in the lead roughers when using ethyl xanthate collector alone. Di-isobutyl dithiophosphinate collector exhibits increased galena flotation rate under the same conditions. It is suggested that the flotation rate of galena is restricted due to the presence of an overlayer of precipitated calcium sulphate detected by XPS. The presence of the calcium sulphate restricts the surface exposure of all the sulphide minerals in the feed to relatively low values The flotation process is selective against minerals which have calcium sulphate predominating at the surface.