The measurements reported below quantify the stability and decay of hydrogen, deuterium, and nitrogen atoms in frozen ammonia above 100 K. The decay of H atoms is observed on a time scale of minutes in the range of 100-110 K and follows first-order kinetics. Analogous decays of D and N atoms are observed in the ranges 105-120 and 140-160 K, respectively. Activation energies for the decay processes range from 0.1 to 0.4 eV.