Copper electrodeposition has been followed by in situ electrochemical STM on Au(111) electrodes covered by complete decanethiol monolayers. It has been-found that Cu nanoparticles (2-5 nm) were formed at potentials comprising the underpotential deposition (UPD) region on clean gold. The nanoparticle clusters appear to follow a nucleation and sudden growth process as their maximal size is attained instantaneously on the time scale of the STM imaging process. Nanoparticle heights correspond to one atomic layer of Cu. The distribution density of the Cu deposits reaches a maximal value at potentials within the UPD window, as no new formation of clusters nor growth of already existing clusters is seen at potentials well into the bulk deposition potential region. Bulk deposition of isolated Cu nodules is finally seen at potentials 200 mV negative of the Nernstian potential for Cu reduction, probably resulting from thiol film breakdown. Moreover, nanoparticles remain on the Au surface at potentials as high as 1000 mV positive of the equilibrium potential. Passivation of the nanoparticles is proposed to explain these observations.