Ammonia borane NH3BH3 is a well-known thermolytic hydrogen storage material. However, the mechanism of dehydrogenation under heating is very complex. In this context, we have studied the thermolytic dehydrocoupling of solid, partially deuterated, ammonia borane NH3BD3 up to 200 °C, that is, up to the release of the second equivalent of hydrogen, by using thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry. Our results show that the process, resulting in the release of hydrogen (i.e., HD), is mainly driven by heteropolar hydrogen-deuterium interactions (N-Hδ+···δ-D-B). Homopolar dihydrogen interactions (N-Hδ+···δ+H-N) appreciably contribute to hydrogen (i.e., H2) release. In contrast, the contribution of homopolar dideuterium interactions (B-Dδ-···δ-D-B) is negligible. In summary, this work sheds new light on the mechanism governing the release of hydrogen from ammonia borane under thermolytic conditions.