A phenomenon was found in electrophoretic deposition (EPD) of Au nanoparticles: the ordered two-dimensional (2D) assembly of Au nanoparticles can take place not only on the surface of electrode, but also in the bulk hydrosol. Cu2+ generated from copper anode during EPD has a major effect on the assembly in sol of Au nanoparticles. (c) 2007 The Electrochemical Society.