Accurate measurements of the volume change of the synthetic garnets pyrope and almandine were performed under hydrostatic compression up to about 100 kbar by means of the X ray diffraction technique using a cubic anvil type of high-pressure apparatus. A least squares calculation using the Birch-Murnaghan equation gives the isothermal bulk modulus KT=1.71¿0.03 Mbar for pyrope and KT=1.75¿0.07 Mbar for almandine.