The pressure-volume-temperature (P-V-T) equation of state (EoS) of synthetic grossular (Grs)-andradite (And) solid-solution garnet sample have been measured at high temperature up to 900 K and high pressures up to 22.75 GPa for Grs(50)And(50), by using in situ angle-dispersive X-ray diffraction and diamond anvil cell. Analysis of room-temperature P-V data to a third-order Birch-Murnaghan (BM) EoS yields: V (0) = 1706.8 +/- 0.2 (3), K (0) = 164 +/- 2 GPa and K' (0) = 4.7 +/- 0.5. Fitting of our P-V-T data by means of the high-temperature third-order BM EoS gives the thermoelastic parameters: V (0) = 1706.9 +/- 0.2 (3), K (0) = 164 +/- 2 GPa, K' (0) = 4.7 +/- 0.2, (a,K/a,T) (P) = -0.018 +/- 0.002 GPa K-1, and alpha (0) = (2.94 +/- 0.07) x 10(-5) K-1. The results also confirm that grossular content increases the bulk modulus of the Grs-And join following a nearly ideal mixing model. The relation between bulk modulus and Grs mole fraction (X (Grs)) in this garnet join is derived to be K (0) (GPa) = (163.7 +/- 0.7) + (0.14 +/- 0.02) X (Grs) (R (2) = 0.985). Present results are also compared to previously studies determined the thermoelastic properties of Grs-And garnets.