We have developed a method for controlled generation and in situ observation of nanoscopic metal clusters (Cu) on solid substrates: high-energy electron irradiation of weakly bonded salts (CuCl2). Employing the primary electron beam of a scanning electron microscope as a metal-generating probe, we can initiate, observe, and control the nucleation and further evolution of Cu clusters on a substrate covered with a layer of solid CuCl2. Agglomerates of metal clusters with lateral dimensions below 10 nm can be "written" by this method, e.g., to carry out diffusion experiments. Using this method, we have found strong evidence for the existence of a growth mechanism of metal particles that has not been described before in the literature: incorporation of metal atoms at the metal-substrate interface. (C) 2004 American Vacuum Society.