The electrochemical detection of mercury in aqueous solutions was studied at glassy carbon (GC) and indium-tin oxide (ITO) electrodes modified by gold nanoparticles (Au NPs). Two methods of modification were used: electrochemical reduction of HAuCl4 and electrostatic adsorption of Au NPs stabilized by citrate. We found that the Au NPs modified surfaces yielded higher sensitivity and sharper and more reproducible stripping peaks of Hg as compared with the bare electrodes. The effect of the modification by Au NPs on the stripping potential was examined. Interestingly, the stripping of Hg on GC and ITO modified by Au NPs occurred at the same potential as on bare GC and ITO, respectively. Only the full coverage of ITO by either electrochemically deposited Au for a long time or by vapor deposition, shifted the stripping potential more positive by ca. 0.4 V to that observed on a bare Au electrode. These and further experiments led us to conclude that the Au NPs served as nucleation sites for the deposition of Hg, whereas the GC or ITO are superior for the stripping of mercury. Hence, a combination of well-defined Au NPs on ITO or GC were found ideal for the electrochemical detection of Hg. Indeed, we achieved a remarkable detection limit of 1 μm·L(-1) of Hg using an ITO surface modified by electrostatically adsorbed Au NPs.