The thermoelastic parameters of synthetic Ca3Al2Si3O12 grossular garnet were examined in situ at high-pressure and high-temperature by energy dispersive X-ray diffraction, using a Kawai-type multi-anvil press apparatus coupled with synchrotron radiation. Measurements have been conducted at pressures up to 20 GPa and temperatures up to 1,650 K: this P, T range covered the entire high-P, T stability field of grossular garnet. The analysis of room temperature data yielded V-0,V-300 = 1,664 +/- 2 angstrom(3) and K-0 = 166 +/- 3 GPa for K'(0) fixed to 4.0. Fitting of our P-V-T data by means of the high-temperature third order Birch-Murnaghan or the Mie-Gruneisen-Debye thermal equations of state, gives the thermoelastic parameters: (partial derivative K-0,K-T/partial derivative T)(P) = -0.019 +/- 0.001 GPa K-1 and alpha(0,T) = 2.62 +/- 0.23 x 10(-5) K-1, or gamma(0) = 1.21 for fixed values q(0) = 1.0 and theta(0) = 823 (Isaak et al. Phys Chem Min19:106-120, 1992). From the comparison of fits from two different approaches, we propose to constrain the bulk modulus of grossular garnet and its pressure derivative to K-T0 = 166 GPa and K'(T0) = 4.03-4.35. Present results are compared with previously determined thermoelastic properties of grossular-rich garnets.