New results of an investigation of NH3BH3 dehydrogenation with supplying a limited quantity of aqueous solution of a catalyst precursor to a solid-state bed of the hydride particles with subsequent external heating at 40–90 °C are presented. Measurements of the reaction layer temperature and the amount of the evolved hydrogen have shown that at external heating temperature higher than 85°С there was acceleration of the first stage of the process, the highly exothermic catalytic hydrolysis of a portion of ammonia borane, which resulted in a stronger heating of the reaction layer and the start of NH3BH3 thermolysis. This type of process is referred to as hydrothermolysis. A TEM, ATR FTIR, and XRD investigation has shown that in the reaction medium the metal chlorides become reduced to an amorphous catalytically active phase. During this process, ammonia reacts with chlorides to form NH4Cl. All of this leads to increased rate of hydrogen generation and hydrogen yield. Gravimetric hydrogen capacity of 7.6 wt% and the average rate of H2 evolution of 39 ml·gcomp. −1min−1 have been achieved at molar ratios of NH3BH3/MCl2 = 50 (M = Co, Ni) and H2O/NH3BH3 = 2 and at external heating of 85°С.