The elastic wave velocities of a dense polycrystalline specimen (99.7% of theoretical density) of synthetic grossular garnet (Ca 3 Al 2 Si 3 O 12 ) were measured to pressures of ∼10GPa and temperatures of 1000K by transfer-function ultrasonic interferometry in conjunction with energy-dispersive synchrotron X-radiation in a deformation DIA-type cubic-anvil apparatus. The calculated elastic bulk (K s ) and shear (G) moduli data were fitted to functions of Eulerian strain to 3rd order, yielding the zero-pressure values [K s =171.2 (8)GPa; G=107.4 (2)GPa] and their pressure derivatives [(∂K s /∂P) T =4.47 (2); (∂G/∂P) T =1.29 (5)]. The temperature dependences of the elastic moduli obtained from linear regression of entire P–T–K s and P–T–G data are: (∂K s /∂T) P =−1.38 (3)×10 −2 GPa/K and (∂G/∂T) P =−1.28 (2)×10 −2 GPa/K. These results together with those from previous studies for garnets with varying compositions suggest that most of the thermo-elastic properties of garnet are insensitive to grossular content, with the exception of the shear modulus, which significantly depends on the calcium content.