This proposal seeks to investigate the effect of temperature on the structures of thallium and caesium azide. High-resolution neutron powder diffraction data are necessary to allow the definitive structure solution of the low-temperature form of TlN3, for which we have already collected complementary X-ray powder diffraction data. Furthermore, the high-temperature behaviour of both TlN3 and CsN3 is to be investigated. The high quality of data expected will not only facilitate the structure determination of their high-temperature forms, but will also provide an insight into a proposed disorder mechanism that precedes the high-temperature phase transition. The results will contribute to the rich structural chemistry that is emerging for metal azides, and will shed light on structure-property relationships of this important class of energetic materials.