Summary: Pentacrythritol tetranitrate has been examined with X-rays by the Bragg ionization spectrometer method. It has been found to be built on the simple tetragonal lattice Г_t, to belong to the space-group D_4h⁷, and to have four molecules per unit cell. The molecule is believed to have one simple digonal axis parallel to the axis c, and two planes of symmetry parallel to (100) and (010) intersecting in it. There is considerable departure from the tetrahedral angle (109° 28′) between the bonds from the central carbon atom, the values of the angles between these bonds being two of them 126° 27′ and four of them 101° 40′. The arrangement of the atoms in the nitro-group is thought to be −N_−O^−O with trivalent nitrogen, or −N_=O^=O with pentavalent nitrogen, and not - N = O = O. It is thought that the strain due to the departure from the tetrahedral angle of the bonds from the central carbon atom of the molecule, and the comparative proximity of eight oxygen atoms at intervals in the structure, and the consequent repulsion, may be contributory causes of the explosive nature of the compound.