The knowledge about how CO occupies and detaches from specific surface sites on well-structured Pt surfaces provides outstanding information on both dynamics/mobility of COads and oxidation of this molecule under electrochemical conditions. This work reports how the potentiostatic growth of different coverage CO adlayers evolves with time on both cubic and octahedral Pt nanoparticles in acidic medium. Data suggest that during the growth of the CO adlayer, COads molecules slightly shift toward low coordination sites only on octahedral Pt nanoparticles, so that these undercoordinated sites are the first filled on octahedral Pt nanoparticles. Conversely, on cubic Pt nanoparticles, adsorbed CO behaves as an immobile species, and low coordinated sites as well as (100) terraces are apparently filled uniformly and simultaneously. However, once the adlayer is complete, irrespectively of whether the CO is oxidized in a single step or in a sequence of different potential steps, results suggest that COads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles. ; M. J. S. Farias is grateful to the CAPES/PNPD (Brazil) for the financial support. G. A. Camara acknowledges CNPq (grants no. 405695/2013-6 and 309176/2015-8) and FUNDECT (grants no. 23/200.583/2012). J. M. Feliu thanks the MINECO (Spain) project CTQ2016-76221-P. J.S.G. acknowledges financial support from VITC (Vicerrectorado de Investigación y Transferencia de Conocimiento) of the University of Alicante.