The electrochemical reaction of NH4+ and mercury has been studied at temperatures below the freezing point of mercury for the first time using potential step and linear sweep techniques. It was demonstrated that the liquid-solid transition of mercury plays a central role in the stability and electrochemical reversibility of ’amalgam’. The ’ammonium amalgam’ is deposited at the cathode surface as a solid phase. The deposition process proceeds through the nucleation and growth step, and the product was stable for at least several minutes. Electrochemical properties of the reduction product of NH4+ and mercury discovered at temperatures below the freezing point of mercury, place ’ammonium amalgam’ in the group of R4N-metals. We concluded that within the electrogenerated solid phase, ammonium-mercury, the electrons are associated with mercury in the form of negative clusters while NH4+ acts as a counterion.