NaBH4 does not absorb NH3 below 100 kPa but transforms into a liquid state after NH3 absorption. On the other hand, LiBH4 absorbs NH3 at pressures lower than 100 kPa. Interestingly, mixed borohydrides absorbed NH3 at low pressures and were liquefied above 100 kPa due to a synergetic phenomenon. The kinematic viscosity of the liquefied state was in situ analyzed during NH3 absorption.