The non-stoichiometric sulfide mineral pyrrhotite (Fe(1-x)S),common to many nickel ore deposits, occurs in differing crystallographic forms and compositions. The processing of pyrrhotite from these ores through froth flotation is based on the surface properties of the sulfides and since pyrrhotite is a metallic conductor, it is of interest to characterise the surface properties of pyrrhotite with respect to its electrochemical state. In this study, a series of pyrrhotite samples derived from Canada, South Africa, and Botswana whose mineralogy is well characterised, were used for electrochemical impedance spectroscopy (EIS). The behaviour of the different pyrrhotite samples were compared in terms of the effect of pH (7 and 10), collector addition (SIBX) and copper activation and the results correlated with microflotation tests. The EIS results were then used to interpret and understand the differences in flotation performance of the pyrrhotite samples under the different reagent conditions and provide some answers as to why the success of copper activation on pyrrhotite is so variable. (C) 2010 Elsevier Ltd. All rights reserved.