The electrocatalytic behaviour of copper in acid is dominated by a metastable state interfacial redox transition which occurs within the double layer region, at ca. -0.7 (+/- 0.1) V (SMSE). The transition in question is apparently based on the presence of low coverage, redox active, copper surface clusters which function as active sites or mediators and thus facilitate electron transfer across the interface; copper in acid exhibits a remarkably high overpotential of ca. 1.2 V for oxygen reduction. The effect of copper plating bath additives on the electrocatalytic properties of copper was surveyed and it was demonstrated that with a compound such as thiourea, the presence in solution of a species such as nitrate, which under additive-free conditions undergoes rapid mediated reduction, exacerbates the surface deactivating effect of the additive.