The effect of sulphite interaction with galena on the mechanism of ethyl xanthate adsorption onto galena surfaces has been studied in situ using UV spectroscopy. The influence of sulphite on galena flotation has been studied. X-ray photoelectron spectroscopic (XPS) and dissolution studies have been used to identify the mechanism of interaction between sulphite and galena surfaces. Metastable ethyl monothiocarbonate is a significant derivative of ethyl xanthate adsorption on galena in the absence of sulphite. The concentration of surface sites which produce monothiocarbonate increases with the initial state of galena oxidation, which, is in turn, apparently dependent upon galena preconditioning time, temperature and oxygen concentration. Significantly different ethyl xanthate adsorption characteristics were found, after sulphite conditioning of the galena surface. In this case, ethyl xanthate adsorption rate was significantly reduced, as was the formation of monothiocarbonate. Surface complexation of lead hydroxide by adsorbed sulphite decreases the rate of ethyl xanthate adsorption onto sites which, potentially, can produce monothiocarbonate. Galena dissolution studies and XPS examination of the galena surfaces together, confirmed the formation of insoluble lead sulphite precipitates at the galena surface. The effectiveness of sulphite depression of galena flotation is enhanced by adsorbed lead hydrolysis products on galena.