According to the mixed potential model for flotation of sulphide minerals with xanthates, oxygen reduction constitutes the cathodic reaction balancing the oxidation of xanthate. The aim of the present work was to determine the mechanism of oxygen reduction at galena and pyrite in the presence of xanthate. As a comparison two inert metals were also studied. The rotating ring disc electrode technique was used to determine the amount of hydrogen peroxide produced at the different materials. Chronoamperometry and cyclic voltammetry were used to study the adsorption properties of xanthate at platinum, used as ring material for the RRDE＇s. The oxygen reduction is in all cases inhibited in the presence of xanthate. At galena, platinum and gold the reduction involves only two electrons and hydrogen peroxide can be detected at the ring for gold and platinum in alkaline solution. Further reduction of hydrogen peroxide may take place in parallel to hydrogen evolution. Pyrite is least influenced by the presence of xanthate and the oxygen reduction proceeds with a total of four electrons. In alkaline solution hydrogen peroxide can be detected at the ring electrode. Most interesting is the fact that hydrogen peroxide is formed as a stable product in a large potential region at platinum in the presence of xanthate. This is explained by the adsorption of xanthate at the surface, as revealed from cyclic voltammetry and chronoamperometry. The strong adsorption of xanthate on the platinum surface complicates the analysis of the RRDE data. The homogeneous reaction between hydrogen peroxide and xanthate, forming perxanthate was studied in alkaline solution by UV spectrophotometry. It was concluded that the reaction is too slow to effect the determination of hydrogen peroxide at the ring electrode.