Surface potential of pyrite in an aqueous environment was measured by means of a single crystal pyrite electrode. The effect of the activity of S2- and Fe2+ ions as well as of pH, was examined. The results show that S2- and Fe2+ ions are the dominant potential determining ions. These ions are bound to surface sites with the extent depending on their activity in the bulk of the solution. Adsorption affinity of S2-ions is significantly higher than that of Fe 21 ions so that in most cases the pyrite surface is negatively charged. The results were explained on the basis of the surface complexation model. It was found that the surface potential depends also on pH. (c) 2007 Elsevier B.V. All rights reserved.