AbstractExpansion of TATB is studied on a molecular level by means of x‐ray crystallography. Continuous monitoring of the cell constants of TATB between 214 K and 377 K allows calculation of a volume change of +5.1% for this molecular system. Expansion of the pure material is almost exclusively a function of a 4% linear increase in the c axis (the perpendicular distance between sheets of hydrogen‐bonded TATB). Calculated from these data, the volume coefficient of thermal expansion for crystalline TATB is 30.4 × 10−5K−1. The structural features of crystalline TATB and its anisotropic thermal‐expansion behaviour are compared with those of graphite and boron nitride. Two other crystalline products in the bulk TATB are either actual polymorphs of TATB or