The Li-Mg-N-H system composed of 3Mg(NH2)(2) and 8LiH reversibly desorbs/absorbs similar to 7 wt % of H-2 at 120-200 degrees C and transforms into 4Li(2)NH and Mg3N2 after dehydrogenation. In this work, the mechanism of the hydrogenation reaction from 4Li(2)NH and Mg3N2 to 8LiH and 3Mg(NH2)(2) was investigated in detail. Experimental results indicate that 4Li2NH is first hydrogenated into 4LiH and 4LiNH(2). At the next step, 4LiNH(2) decomposes into 2Li(2)NH and 2NH(3), and the emitted 2NH(3) reacts with (1/2)Mg3N2 and produces the (3/2)Mg(NH2)(2) phase, while the produced 2Li(2)NH is hydrogenated into 2LiH and 2LiNH(2) again. Such successive steps continue until all 4Li(2)NH and Mg3N2 completely transform into 8LiH and 3Mg(NH2)(2) by hydrogenation.