The presence of sulphide ions in solution greatly influences the cathodic behaviour of copper in borax solutions (pH 9.2), even when this contaminant is in low concentration. Cyclic voltammograms, polarisation curves and Tafel plots are compared for polluted and unpolluted solutions. Prereduced and aged copper electrodes are investigated. The presence of sulphide ions in solution increases the rate of oxygen reduction on copper, thus having a detrimental effect on the global corrosion process. The transformation of the surface film into one with better conductive properties is suggested.