Comprehensive thermodynamic calculations have been carried out on the chalcocite-KEX-water system based on complete mass balance equations which include both soluble and insoluble species. The calculations have yielded i) E_h-pH stability diagrams for different KEX additions, ii) equilibrium concentrations and amounts of all the dissolved and insoluble species including those of CuX and CuX₂, iii) two- and three-dimensional plots showing the effect of E_h and pH on the formation of selected species, and iv) minimum xanthate additions required to form CuX and CuX₂. These information can be used as a guide in predicting the optimum conditions for flotation and leaching of chalcocite. The upper limiting potentials predicted from the thermodynamic calculations are in excellent agreement with those determined from the microflotation tests. The lower flotation edges, on the other hand, are found to be dependent on the sequence of reagent additions. When xanthate is added after the addition of a reducing or oxidizing agent to control the potential, they are in reasonable agreement with predicted values. When the collector is added prior to the potential control, however, the lower flotation edges are significantly higher than the predictions. The flotation experiments carried out at several different concentrations show that the minimum amount of the collector is required between 0 to 200 mv, as predicted by the thermodynamic calculations. Voltammetry experiments carried out in the absence of a collector at pH 9.2 and 6.8 suggest that the anodic oxidation of chalcocite results in the formation of Cu₂O, Cu(OH)₂ and S^O. At potentials below -400 mv, Cu₂S is reduced to Cu^O and HS⁻. When xanthate is added, several adsorption peaks are observed. There are indications that the peaks appearing between -200 and -100 mv may involve the reaction between xanthate and Cu^O. However, at potentials above -40 mv, xanthate may adsorb directly on chalcocite without involving Cu^O. / M.S.