The mechanism of localized electrodeposition on the nanometer scale is studied using the tip of an electrochemical scanning tunneling microscope as a "nanoelectrode", which is retracted from the substrate (working electrode) during growth of the clusters. The system chosen exemplarily is Au(111)/Co2+, which shows a relatively weak substrate/deposit interaction compared to the strong interaction characteristics of underpotential deposition systems. The width and height of the clusters, which can be grown with diameters even below 10 nm, are determined by the diameter of the tip apex, the distance between tip and substrate, the substrate potential, and by the amount of Co transferred to the substrate via the tip. The influence of these parameters on the cluster growth can be well understood assuming diffusion as the mechanism of Co transfer from the tip to the substrate. Field and charging effects of either tip or substrate can be excluded due to the large distance of approximately 20 nm between both electrodes.