According to the mixed potential model for flotation of sulphide minerals, oxygen reduction constitute the cathodic reaction. The mechanism and kinetics of oxygen reduction is largely dependent on the properties of the surface. Pyrite is a relatively good catalyst for oxygen reduction and the reaction proceeds through the formation of soluble hydrogen peroxide. Since the surface of pyrite in the flotation pulp can differ from the composition of bulk pyrite it is important to investigate the influence of different surface layers on the oxygen reduction. In the present paper the oxygen reduction was studied using the rotating ring-disc electrode technique. Three different pre-treatments were made to produce (i) an oxidised surface, (ii) a metal deficient surface and (iii) a surface of deposited sulphur. The results show that for all the pre-treatments made the oxygen reduction will be inhibited. The amount of soluble hydrogen peroxide also changes, decreasing in the order pyrite > metal deficient pyrite > oxidised pyrite. For the sulphur covered surface, no soluble hydrogen peroxide could be detected. The results are discussed in the light of flotation of pyrite.