The reactions between zirconium and hafnium metal atoms and ammonia molecules have been studied using matrix isolation infrared absorption spectroscopy and density functional theoretical calculations. The results showed that ground-state zirconium and hafnium metal atoms reacted with NH3 to form the ZrNH3 and HfNH3 complexes spontaneously on annealing. The ZrNH3 and HfNH3 complexes underwent photochemical rearrangement to the HZrNH2, H2ZrNH, and H2HfNH molecules. The reaction intermediates and products have been identified by isotopic substitutions as well as density functional theoretical frequency calculations. In addition, qualitative analysis of the possible reaction paths for these reactions is given, including various minima and transition states.