In copper sulphide flotation, copper adsorbs on pyrite through superficial oxidation of the copper minerals (e.g. chalcopyrite) which promotes pyrite flotation in the presence of xanthate. This "inadvertent" activation of pyrite by copper ions is undesirable in copper sulphide flotation. In order to minimise this effect, depressants are used to suppress the effect of the activating ions. The effect of diethylenetriamine (DETA) in different combinations (under aerated and non-aerated conditions) on Cu-activated pyrite hydrophobised by xanthate was examined using flotation, spectroscopic and solution analyses, at pH 10. The results showed that DETA affects the flotation behaviour and surface chemistry of pyrite. However, high dosages are required. The depression action of DETA on Cu-activated pyrite was attributed to both the removal of surface copper to form soluble Cu-DETA complex in solution and competition for Cu sites on the activated pyrite surface. The significant depression of pyrite in the presence of DETA under the aerated condition was due to increased amount of iron oxy-hydroxides (Fe-O/OH), copper oxides and Cu(I)-DETA hydrophilic species. (C) 2013 Elsevier Ltd. All rights reserved.