The mechanism of copper-activation of sphalerite was studied at pH 9.2. The study was conducted using a carbon matrix composite (CMC) electrode containing sphalerite particles. Rest potential measurement and voltammetry experiments conducted on the CMC electrodes showed that a Cu2S-like activation product was formed when the electrode was activated in 10(-4) M CuSO4 solutions at open circuit. When the electrode was activated at lower potentials, a CuS-like activation product was formed. Sphalerite activated under slightly oxidizing conditions produced hydrophobic species on the surface, possibly copper polysulfides. (C) 2000 Elsevier Science B.V. All rights reserved.