Pt/Sn binary metal electrodes were prepared by the successive adsorption of Sn2+ and PtCl62- ions onto a gold substrate followed 6 by the reduction of the adsorbed PtCl62-. The atomic ratios of Sn and Pt to the surface Au were determined to be approximately 1 and 0.3, respectively, using a quartz crystal microbalance (QCM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. Therefore, binary metal layers at a monolayer level were formed. These Pt/Sn binary metal electrodes exhibited a very high electrocatalytic activity for the reduction of nitrate ions with the predominant formation of ammonium ions.