Abstract The crystal structure of NH 3BH 3 was investigated using synchrotron high pressure X-ray diffraction (HPXRD) up to 27 GPa and neutron diffraction up to 5 GPa. Density functional theoretical (DFT) calculations were carried out simultaneously for comparison. The results confirm a pressure induced phase transition from the tetragonal I 4 mm phase to a high pressure orthorhombic Cmc21 phase around 1.22 GPa. Further increase of pressure above 8 GPa, we observed a second structural transition from Cmc21 to a triclinic P1 phase which are reversible with small hysteresis. The transition pressures and the bulk modulus obtained experimentally are in good agreement with theory.