THE ammonium ion NH4+ behaves in many ways like an alkali metal ion. This led Ramsey1 to propose that monovalent ammonium metal (NH4+ ions immersed in an almost uniform, degenerate sea of electrons) is stable at pressures much lower than typical insulator-metal transition pressures2 of megabars ~1011 Pa (105 Pa = 1 bar). Subsequently, it was estimated3 that the transition from a mixture of NH3 and H2 molecules to metallic ammonium, occurs at a pressure of less than 2.5×1010 Pa. If this estimate were correct, metallic ammonium would be of great interest to experimental high pressure physicists, and to planetary physicists. Indeed, metallic ammonium has been invoked in some4,5 but not all models for the interiors of Uranus and Neptune; and its existence could be of importance6 for calculations of the interior magnetic fields of these planets.