The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H-2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH(2) -> Li2NH + NH3 and LiH + NH3 - LiNH2 + H-2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.