We present an experimental study on 500-3000 eV energy electron irradiation of ammonia ice at 20 K. While molecular nitrogen and diazene (N2H2) were detected during post-irradiation thermal desorption no appreciable traces of hydrazine (N2H4) were observed. We suggest that the recombination between adjacent nitrogen atoms and between neighboring NH radicals can be the main mechanism responsible for the formation of the observed molecules, whereas hydrazine formed by two amino radicals is probably in an excited state and decays into diazene and hydrogen. (C) 2012 Elsevier B.V. All rights reserved.