Infrared spectroscopy encompassing an internal reflection technique has been used to investigate the adsorption products on lead sulphide. The adsorption reactions of potassium ethyl xanthate, sodium diethyldithiophosphate, and potassium diphenyldithiophosphate were studied under controlled pH and potential. Potential control was achieved using sodium sulphide feeding. Both sulphidized and oxidized lead sulphide was used as a starting material. The main type of adsorption product which formed on lead sulphide with each of the collectors resembles the corresponding lead salt, e.g. lead xanthate. A distinct difference, however, can be noted between the precipitated lead salt and the surface product formed on a clean lead sulphide surface. The three different forms that can be observed on the lead sulphide surface are, according to increasing thickness of the collector coverage, monolayer form, surface form, and precipitated form. The precipitated lead salt formed on oxidized lead sulphide surface, but as sodium sulphide was added the precipitated form of the lead salt was converted into the surface form. Adsorption and desorption of collectors was dependent on the potential which was maintained by sodium sulphide feeding. There exists a correlation between sulphide ion concentration and the stability of the collector surface product. It is proposed that the reaction mechanism occurring when the collector adsorbs onto lead sulphide surface is simply an ion exchange reaction mechanism.