Using optical second harmonic generation (SHG) we observe, for the first time, a structural change in the energetic material (dry-aminated production grade) TATB (1,3,5-triamino-2,4,6-trinitrobenzene) upon heating. This structure change is verified by X-ray powder diffraction (XPD). The kinetics of this structure change are measured and are found to closely match commonly accepted thermal decomposition parameters for the first step, analogous to observations made for the beta to delta phase transition in HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine). Consequently, these solid structure transitions appear to be coupled to the chemical decomposition mechanism or it may be the first physical process occurring. We apply this technique as a diagnostic to an ignition experiment. This experiment demonstrates that the structure change can be rapid (it is thermally activated) and that SHG can be used to indicate not only the solid structure change (large increase in SHG intensity) but also the onset to significant decomposition (subsequent drop in SHG intensity). The ability to selectively change the efficiency of the SHG, in a single piece of explosive, is also demonstrated and may have novel nonlinear optical applications. This application of SHG to TATE constitutes a fundamentally new probe in the dynamics of TATE decomposition, ignition, and combustion and may have application to optical integrated devices for frequency conversion and information encoding.