Using powder X-ray diffraction in conjunction with a diamond anvil cell (DAC), the unit cell volume of triamino-trinitrobenzene (TATB) has been measured from ambient pressure to 13 GPa. The resultant isotherm is compared with previous theoretical (Byrd and Rice and Pastine and Bernecker) and experimental (Olinger and Cady) works. While all reports are consistent to approximately 2 GPa, our measurements reveal a slightly stiffer TATB material that reported by Olinger and Cady and an intermediate compressibility compared with the isotherms predicted by the two theroretical works. Analysis of the room temperature isotherm using the semi-empirical, Murnagham, Birch-Murnaghan, and Vinet equations of state (EOS) provided a determination of the isothermal bulk modulus (K-n) and its pressure-derivative (K-a') for TATB. From these fits to our P-V isotherm, from ambient pressure to 8 GPa, the average results for the zero-pressure bulk modulus and its pressure derivative were found to be 14.7 GPa and 10.1, respectively. For comparison to shock experiments on pressed TATB powder and its plastic-bonded formulation PBX 9502 (95% TATB, 5% Kel-F 800), the isotherm was transformed to the pseudo-velocity U-s - u(p) plane using the Rankie - Hugoniot jump conditions. This analysis provides an extrapolated bulk sound speed, c(o) = 1.70 km s(-1), for TATB and its agreement with a previous determination (c(o) = 1.43 km s(-1)) is discussed. Furthermore, our P-V and corresponding U-s - u(p) curves reveal a subtle cusp at approximately 8 GPa. This cusp is discussed in relation to similar observations made for the aromatic hydrocarbons anthracene, benzene and toluene, graphite, and trinitrotoluene (TNT).