The solution interaction of ethyl xanthate and sulphite has been studied by UV spectroscopy. The rate of ethyl xanthate decomposition and formation of an intermediate compound, ethyl perxanthate, has been measured as a function of sulphite concentration, temperature and purging gas. The reaction mechanisms are discussed, with particular emphasis on the role of sulphite in the decomposition of ethyl xanthate and ethyl perxanthate. The regulating effect of ethyl xanthate decomposition by sulphite on ethyl xanthate adsorption on galena is also investigated. A mechanism involving simultaneous ethyl xanthate decomposition and adsorption is proposed to partially explain galena flotation response in the presence of sulphite solution species. The solution interaction of sulphite with xanthate is effectively controlled, allowing the specific effects of sulphite modification of galena surfaces to be more clearly discerned.