The hydrothermolysis of ammonia borane (AB) can release more hydrogen than AB thermolysis itself at mild temperatures. To obtain more insight into the noncatalytic hydrothermolysis, we investigated this process using a high-pressure differential scanning calorimeter and characterized the condensed residue by thermal analysis and infrared spectroscopy. For mass ratios of AB to water of 3.6, 1.44, and 0.72, the first pyrolysis step is not completed in 1 h at 85 degrees C. As the temperature increases from 25 to 115 at 1 degrees C min(-1), there exist two exothermic peaks for the mass ratio of 3.6, centered around 82 and 103 degrees C on the heat flow curve, attributed to the hydrolysis and the first step of AB pyrolysis, respectively. For the mass ratio of 1.44, the top temperatures of these two exothermic peaks are 90 and 100 degrees C, respectively. However, for the mass ratio of 0.72, only the exothermic peak related to hydrolysis is observed, and its top temperature is ca. 106 degrees C. At temperatures up to 115 degrees C, the structural features associated with the product of the second pyrolysis step are absent for the mass ratios of 1.44 and 3.6. In the noncatalytic hydrothermolysis, the hydrolysis occurs much faster than the pyrolysis, and the latter is accelerated by both the heat desorbed from the hydrolysis and its product. Our findings suggest that water prefers reacting with AB rather than binding to the hydrolysis product to form hydrates and the hydrolysis itself can generate about 9 wt % of hydrogen with respect to the total mass of AB and water if sufficient AB is present. In this case, the heat of hydrolysis is 131 kJ (mol of AB)(-1), and the solid hydrolysis product decomposes at temperatures about 350 degrees C.