The crystal structure of the tetragonal phase, V, of NH 4 NO 3 at –150°C has been determined by x-ray methods, by cooling single crystals of the room temperature phase. The axes transformations are: [001] IV –[001] v ; [100] IV and [010] IV –[l[unk]O] v . The lattice constants are a = 7.98 Å, c = 9,78 Å. The space group is P 4 2 and Z = 8. The structure has been solved from the Fourier projections(001)and (1[unk]0). The final values of the discrepancy factor are R hk 0 = 0.15 and R hhl = 0.18.