Acoustic wave velocities for synthetic polycrystalline pyrope (Mg 3 Al 2 Si 3 O 12 ) were measured to 9GPa and temperatures up to 1000°C by ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil DIA-type apparatus (SAM-85). Specimen lengths at high pressures (P) and temperatures (T) are directly measured by X-radiographic methods. Elastic wave travel times and X-ray diffraction data were collected after heating and cooling at high pressures to minimize effect of non-hydrostatic stress on the measurements. A linear fit to the high P and T data set yields the elastic bulk and shear moduli [K S =175 (2)GPa; G=91 (1)GPa] and their pressure and temperature derivatives [K′S=3.9±0.3; G′=1.7±0.2 and (∂K S /∂T) P =−18 (2) MPa/K; (∂G/∂T) P =−10 (1) MPa/K]. In a separate analysis, the pressure–volume–temperature data collected during these acoustic experiments were fit to a high temperature Birch–Murnaghan (HTBM) equation [with K′ fixed at 3.9] and to each isothermal P–V–T data yielding (∂K T /∂T) P =−22 (2) MPa/K and (∂K T /∂T) P =−20 (5) MPa/K, respectively. Comparison of Py 100 data with those other Py–Mj compositions indicates that the thermo elastic properties are insensitive to majorite content in the garnet along the pyrope–majorite join.