Structural determinations of ammonia borane (BH3NH3) have been carried out for the orthorhombic (at 90 K) and tetragonal (at 298 K) modifications using single-crystal X-ray data. The orthorhombic structure (space group Pmn21) agreed with a previously published neutron determination, while the tetragonal structure (I4mm) exhibited halos of hydrogen atom occupancy around both the nitrogen and boron atoms. The bond angles to the regions of hydrogen occupancy are consistent with the expected tetrahedral geometry for –BH3 and –NH3 groups. A new model for tetragonal BH3NH3 was constructed which accounts for the hydrogen disorder in the I4mm structure while introducing only weak new diffraction peaks. These peaks could not be found, however, and it is likely that the hydrogen disorder in tetragonal BH3NH3 arises from either rotations of higher than 3-fold order, or from random orientations of hydrogen-containing groups.