Studies of deposition of CoPt alloys on Pt from neutral or mildly alkaline solutions containing free ammonia indicated that hydrogen evolution takes place under partial mass-transport controlled conditions, at potentials positive with respect to the potential for reduction of water, indicating that NH4+ is the proton donor. This assumption was tested experimentally using platinum electrodes, in the absence of platinum and cobalt ions in solution. Our voltammetric studies indicate that a short-lived intermediate could be formed during the potential sweep in the cathodic direction. This is most probably the Rydberg radical NH40. Although this radical has never been observed in aqueous solution, it may exist in the system studied here, because it can be stabilized by adsorption on the surface. Long plateaus in the plots of the open-circuit potential vs time were observed, following cathodic polarization. These plateaus are attributed to bubbles of molecular hydrogen trapped under the electrode and/or dissolved in the solution adjacent to the surface. (C) 2008 The Electrochemical Society.