Potentiostatic transients of copper electrodeposition on polyorthoaminophenol-Pt modified electrodes were fitted to the three dimensional nucleation and growth model with diffusion control proposed by Scharifker et al. (B. Scharifker, G. Hills, Electrochimica Acta 28 (1983) 879). Non-dimensional plots showed a transition from instantaneous nucleation, at low overpotentials, to progressive nucleation at higher potentials. The formation of nuclei with approximately the same diameter at low potentials and the presence of a major dispersion in the size of the clusters at higher values of overpotential agrees with the fitting. However, discrepancies between the charge consumed during the formation of the copper deposit, at low potential, and that calculated by assuming hemispherical centres suggest that the nuclei are not hemispherically shaped but probably truncated spheres. Nuclei formed at large potential steps are not single centres but cluster of copper around a central site which produces an early overlapping of diffusion fields. The shape of the nucleation centres is explained in terms of electrochemical reduction forming conducting strands in a poorly conducting matrix. The distribution of clusters is fixed by the presence of these conducting chains in the polymer-solution interface.