The stability of the ammonium radical (NH4) was determined from measurements of the kinetic energy released in its fragmentation products following formation in a fast electron capture process: NH4 + + Na → NH4* + Na+. Scattering profiles for heavy (NH3) and light (H) dissociation products were obtained from beam measurements with 5-16 keV NH4+ ions. The existence of a predissociative barrier in the radical is inferred from edge structure and scattering continua in H atom profiles. The radical is bound with respect to a potential minimum but all of the isotopic species NH4, NH3D, NH2D2, and NHD3 undergo rapid loss of H atoms and have ground states lying above their dissociation limits. The radical ND4 has unusual stability with its ground state lying close to or below its dissociation limit. Dissociative lifetimes for stable and unstable states of ND4 differ by at least two orders of magnitude. The possible significance of these observations on the interpretation of optical transitions involving the ground states of NH4 and ND4 are discussed. From these measurements the ionization potential of 4.73±0.06 eV for NH4 is calculated. The stabilities of CH 3NH3 and CH3ND3 radicals and their dissociative pathways have also been investigated.