A widespread use of PEM fuel cells is hindered by the absence of a practical, high density hydrogen fuel source. Among various alternatives, chemical methods of hydrogen storage provide high specific energy at relatively easy storage conditions. Ammonia borane (AB) is a promising hydrogen storage material as it contains 19.6 wt% hydrogen. To release hydrogen from AB, thermolysis and catalytic hydrolysis are usually employed, but these methods require either high temperatures or expensive catalysts. Hydrothermolysis of AB, investigated in the present work, allows for hydrogen generation (from both AB and water) at moderate temperatures and without the use of catalyst. This method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure (~ 10 bar) to avoid water boiling. To clarify the reaction mechanism, D2O was used instead of H2O in certain experiments. It was shown that heating AB/D2O solution to temperatures 117-170 °C releases 3 equiv. of hydrogen per mole AB, where 2-2.1 equiv. originate from AB and 0.9-1 equiv. from water. The results indicate that the proposed method is promising for hydrogen storage, especially for applications where off-board water is used.