Chitosan, a natural polymer extracted from crustacean shells, was found to selectively depress chalcopyrite while galena was floated by xanthate. At pH 4, the recovery of galena was up to 95% while that of chalcopyrite was 30% when flotation tests were carried out on mixtures of chalcopyrite and galena. However, in single mineral flotation and adsorption tests, it was found that both chalcopyrite and galena were depressed and chitosan adsorbed on both minerals. The observed selectivity seemed to have originated from competitive adsorption. ToF-SIMS measurements indicated that when chalcopyrite and galena were present together in the suspension, chitosan barely adsorbed on galena but adsorbed heavily on chalcopyrite. Chitosan-metal ions adsorption test, X-ray photoelectron spectroscopy (XPS) as well as ToF-SIMS were employed to study the interaction of chitosan with chalcopyrite and galena and to delineate the observed competitive adsorption. Chitosan could potentially be used in differential Cu-Pb sulfide separation to replace the hazardous inorganic depressants such as cyanide, dichromate and sulfur dioxide. (C) 2012 Elsevier B.V. All rights reserved.