The effect of pressure on the lattice parameter, and hence on the volume, of two samples of grossularite garnet (Ca3Al2Si3O12) has been measured at room temperature up to 250 kbar with the use of a diamond anvil high-pressure X ray diffraction camera. The pressure to which the sample was subjected was measured by means of the NaCl internal pressure standard. A least squares fit of the pressure-volume to the Birch-Murnaghan equation yields a bulk modulus value of K0=1.74¿0.04 Mbar when a (∂K/∂P) ‖P=O value of 4.25 (Halleck, 1973) is used. This K0 value is in agreement with the value of 1.704 Mbar obtained by Halleck <1973>, using ultrasonic techniques. A comparison of the bulk moduli of grossularite, pyrope, and almandite shows that K0∝V0-1 for Ca and Mg garnets as expected from static lattice models. On the other hand, K0∝V0-5 for the Mg-Fe garnet solid solutions. This behavior has not been explained by available theories.