Stirred mills have been widely used for regrinding and are more energy efficient than tumbling mills. These two types of mills present different particle breakage mechanisms and redox environments during grinding. In this study, the effect of regrinding with these two types of mills on the separation of chalcopyrite from pyrite in the cleaner stage was studied. A laboratory rod mill and a laboratory stirred mill were used to regrind rougher flotation concentrates. It was found that chalcopyrite and pyrite exhibited different flotation behavior after regrinding with the rod mill and the stirred mill, resulting in different separability of chalcopyrite from pyrite. The mechanism underpinning this phenomenon was investigated by a range of techniques including dissolved oxygen demand measurements, X-ray photoelectron spectroscopy (XPS) and Time of flight secondary ion mass spectrometry (ToF-SIMS). It was found that the two mills produced different surface oxidation and pyrite activation by copper ions which determined the separation of chalcopyrite from pyrite. This study demonstrates that the selection of a regrind mill should not only depend on its energy efficiency but also the property of surfaces produced for subsequent flotation. (C) 2015 Elsevier Ltd. All rights reserved.