Carbon supported iridium-octaethylporphyrin (IrOEP), iridium-tetraphenylporphyrin (IrTPP) and iridium-phthalocyanine (IrPc) were studied in acid for the oxygen reduction. Both porphyrins give a four-electron reduction, although a peculiar deactivation at low potentials occurs. At IrPc and heat treated porphyrins hydrogen peroxide is formed. Results are compared with carbon monoxide oxidation experiments at these catalysts. A single site mechanism is proposed for the oxygen reduction mechanism.