The anodic oxygen evolution on Cu, Co and Mo was investigated under potential control in flow-through micro cells by fluorescence quenching of dichlorotris(1,10-phenanthroline)ruthenium(II)hydrate. The experiments were carried out in concentrated chloride and nitrate solutions up to extreme current densities of 70 A/cm(2). The amounts of oxygen were compared to the anodic charge and yielded oxygen efficiencies. In concentrated nitrate electrolyte, this efficiency is >70% for Cu around 1 A/cm(2) and decreases to less than 20% at larger current densities, indicating strong Cu dissolution. The reduction of oxygen is due to the formation of supersaturated product films with low water content. Co dissolves in active state without oxygen evolution up to 10 A/cm(2) but then shows some tendencies of passivation with oxygen evolution. Mo dissolves as molybdate with 5% oxygen in nitrate and even less in chloride. (c) 2012 Elsevier Ltd. All rights reserved.