The surface properties of enargite as a function of pH were investigated, The zeta potential values' increased from - 10 mV (pH = 3) to - 45 mV (pH = 12) and no isoelectric point (iep) was detected. The zeta potential of enargite was highly sensitive to pH and interesting peaks/valleys, unusual in common sulfide minerals, were observed. In the present study, these results were interpreted on the basis of the chemistry of enargitc, which is a tetra-thioasenate of Cu(I), A thermodynamic approach, including the Eh-pH diagram for enargite, and the hydrolysis diagrams for arsenic, thioarsenic, and oxo-thioarsenic acids was employed. An interpretation based on the simultaneous ionization and dissociation reactions of the hydrolysis products of arsenic and thioarsenic acids on the enargite surface was discussed, On the other hand, covellite (CuS), under low concentrations of arsenate ions, showed similar zeta potential peaks. Additionally, enargite's zeta potential showed sign reversal for a certain pH range, with the addition of low concentrations of Cu(I) and Cu(II) ions, suggesting that other hydrolysis and precipitation reactions may also be involved. (c) 2005 Elsevier B.V. All rights reserved.