In this study, the mechanism which underpins the different interactions of chalcopyrite, a primary copper sulfide mineral, and chalcocite, a secondary copper sulfide mineral, with bentonite, a clay mineral, during the grinding with stainless steel media was investigated by a range of techniques including XPS (X-ray Photoelectron Spectroscopy) analysis, electrokinetic studies and Cryo-SEM (Scanning Electron Microscope) measurements. It was found that during grinding chalcocite was heavily oxidized but chalcopyrite was slightly oxidized. As a result of the different extent of mineral surface oxidation, chalcopyrite remained negatively charged but chalcocite became positively charged after grinding leading to the different interactions with bentonite particles. This study provides a guideline to improve the extraction of copper sulfide minerals in the presence of clay minerals, an important issue in mineral processing. (C) 2012 Elsevier B.V. All rights reserved.