The electroreduction of molecular oxygen is investigated between -0.1 and -0.5 V vs SHE on bornite, Cu5FeS4, at pH 9.2 and 14, by means of cyclic voltammetry (CV) and stationary voltammetry (SV), using a double channel electrode flow cell (DCEFC). Using an E/pH diagram established in this work, the CV results suggest that the bornite surface is stable between -0.1 and -0.5 V then oxidized to CuS and Fe(OH)(3) above -0.1 V whereas, below -0.5V the mineral reduces to metal sulphides: Cu2S and FeS. The SV results show that oxygen is reduced to peroxide ions, HO2-. At pH 9.2 the generated sulphide ions hinder the oxidation of HO2- on the collector electrode of the DCEFC, due to the formation of a blocking surface layer of elemental sulphur, S, impeding the determination of the kinetic parameters, k(1) (direct way) and k(2) (indirect way) of the oxygen electroreduction reaction. In contrast, at pH 14, as soluble polysulphides are formed, it was possible to determine these parameters, showing that the bornite is a poor catalyst for oxygen reduction. At pH 14, in the presence of potassium ethylxanthate, generally used as a flotation collector, the ethylxanthate ions, C2H5OCSSO-, are oxidized by HO2- to perxanthates, ROCSSO-, while at pH 9.2 the oxygen reduction is inhibited due to ethylxanthate chemisoption on the bornite surface.