Certain natural. waters appear to contain copper sulfide (CuxS) nanoparticles in nanomolar concentrations (as Cu). These nanoparticles have been tentatively identified by the characteristic pH below which they deposit sulfide onto Hg electrodes. A proposed alternate approach to studying CuxS nanoparticles relies on their hydrophobicity, which causes them to sorb to Hg electrodes; there they can undergo reduction at -0.9 to - 1.1 V vs. Ag/AgCl. However, solutions supersaturated with respect to Cu sulfide phases also form CuxS directly at Hg electrode surfaces. The voltammetric reduction peaks obtained from these deposits are not clearly distinguishable from those obtained from sorbed nanoparticles. Surface formation of CuxS, which appears to be limited to approximately two layers, involves a reaction between Cu amalgam and electrodeposited HgS. Surface-formed CuxS could be problematic in studies of CuxS nanoparticles, but this obstacle can be avoided by conducting voltammetric accumulations at potentials too negative for HgS electrodeposition (e.g. -0.85 V). Electroreduction of surface-formed CuxS occurs by a two-dimensional instantaneous hole nucleation and growth process. (c) 2006 Elsevier B.V. All rights reserved.