The electrocatalytic activities of two kinds of iridium oxide electrodes are studied for their ability to evolve hydrogen and oxygen in 1 M H2SO4 at room temperature. The first kind of electrode is made of anodic iridium oxide nanoparticules (AIRONP) prepared by cycling well-defined iridium metal nanoparticles supported on carbon between O-2 and H-2 evolution potentials. The oxidation process can be followed by cyclic voltammetry and CO oxidation experiments. From TEM observation and the knowledge of the active layer loading, the active surface area of these electrodes are estimated and their activities are presented for the two reactions. For the sake of comparison, electrodes fabricated by thermal decomposition of an iridium salt (TOIROF) have been studied. Their electroactive real surface area have been determined from voltammetric charge measurements. Beforehand, a correlation factor between the real surface area measured by gas adsorption experiments and this later parameter had been established. The mechanism for the two evolution reactions and for the two different electrodes is discussed in the light of our results and literature data.