In this paper we have studied how the deposition of platinum adatoms at gold electrode surface in undetectable amount by observation of the gold electrode CV profile may put in evidence the first stage of the appearance of a catalytic activity for a material initially inactive. In this respect, we will show that the electrochemical hydrogen oxidation reaction (HOR) is an extremely sensitive surface probe for detection of very small amount of surface active sites such as platinum adatoms below 10(-4) ML. The activity of the site is revealed by its ability to oxidize hydrogen from the solution in contrast with the gold substrate. The present results open new perspectives for improving the description of the hydrogen redox reaction in particular by a strong decrease of diffusion limitation. It results a possibility to put in evidence for the HOR the existence of two polarization regions as a consequence of two consecutive steps. At low overpotential, the electronic transfer is the slow step, whereas for higher overpotential values the hydrogen adsorption on the active sites becomes the rate limiting step as a consequence of the competitive adsorption with the solution components. (C) 2012 Elsevier Ltd. All rights reserved.