In the current work the concept of the active site is analyzed. With this end, Pt was deposited on Au(111) electrode assisted by water in oil microemulsion, in which the micelles confine the formation of small Pt clusters. The presence of highly and lowly coordinated Pt atoms was finely tuned by controlling the annealing processes and confirmed by CO stripping voltammetry in conjunction with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). This procedure allowed for the individuation of the effects of materials free and rich of catalytic defect sites on the CO electrochemical oxidation profile. Thus, a clear correlation between low-coordinated Pt atoms and the appearance of CO oxidation peaks at more negative potentials (peaks at 0.26 and 0.59 V vs. RHE) compared to high-coordinated Pt atoms (ca. 1.1 V vs. RHE) was identified. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.