Solution chemistry, zeta potential and Fourier transform infrared spectroscopic measurements were performed to study the mechanisms of selective depression of sulfide minerals by thioglycollic acid/citric acid during the flotation of carbonate minerals by sodium oleate. It was observed that the addition of the mixed depressants (thioglycollic acid and citric acid) removed active sites from pyrite surfaces and competed for adsorption sites on chalcopyrite surfaces, thus lowering the adsorption of sodium oleate and depressing the flotation of these sulfide minerals. On the other hand, the adsorption of the mixed depressants and sodium oleate on the carbonate minerals (calcite and dolomite) was not mutually exclusive thus flotation of the carbonate minerals was possible in the presence of the mixed depressants. The role of citric acid in the mixed depressant seemed to be that of complexation of the copper ions, thus lowering their activity to catalyze the oxidation of thioglycollic acid. The oxidation product, dithiodiglycollic acid, seemed to be a less effective depressant than the thioglycollic acid. (C) 2004 Elsevier Ltd. All rights reserved.