The oxidation of synthetic and natural samples of tennantite and enargite is compared by measuring the dissolution and the zeta potential of these minerals. The changes in zeta potential with pH and oxidizing conditions are consistent with the presence of a copper hydroxide layer covering a metal-deficient sulfur-rich surface and with the extent of this copper hydroxide coverage increasing with oxidation conditions, Analysis of the zeta potential data reveals that during the acid titration of the minerals, dissolution of the surface copper hydroxide layer occurs at neutral pH values, whereas precipitation of copper hydroxide on the mineral surface is observed during the base titration. Hysteresis between the zeta potential acid and base titration curves is only observed in oxidizing conditions and is attributed to the dissolution of copper from the mineral lattice at acidic pH values. Arsenic does not appear to contribute to the mineral oxidation; in particular, its dissolution is much less than that of copper. This study has shown that in alkaline conditions the natural samples of tennantite and enargite oxidize more than the synthetic samples and tennantite oxidizes more extensively than enargite.