We have studied Ir electrodes in H2SO4 and HClO4 aqueous solutions by cyclic voltammetry between the hydrogen evolution region and the oxygen evolution region. Special attention has been given to the prepeak between the double-layer region and the main oxidation peak. The influence of several parameters, particularly the nature of the electrolyte, on these two peaks was studied. A mechanism that accounts for the prepeak and for the role played by the anions is proposed.