The reaction of NH3 With (OH)-O-. or SO4.- radicals produces the aminyl radical, (NH2)-N-.. Pulse radiolysis and laser flash photolysis techniques were utilized to study the formation of this radical, its absorption spectrum, its reaction with O-2, and the mechanism of formation of subsequent intermediates and the main final product, peroxynitrite. The rates of formation of (NH2)-N-. and its absorption spectrum are in agreement with previous reports. The reaction of (NH2)-N-. with O-2, however, was observed to take place much more rapidly than reported before and to involve an equilibrium of these reactants with the aminylperoxyl radical, NH2O2.. The equilibrium is shifted toward completion of the reaction via catalyzed decomposition of this peroxyl radical, and this decomposition affects the observed rate of reaction of (NH2)-N-. with O-2. The peroxyl radical deprotonates and isomerizes and finally forms NO. In the presence of O-2(.-), NO is converted rapidly to peroxynitrite, ONO2-. This product, which is stable at alkaline pH, was confirmed by gamma-radiolysis of aerated ammonia solutions.