End-member synthetic katoite and grossular were studied by energy-dispersive X-ray diffraction in a diamond anvil cell to 42.3 GPa and 18.4 GPa, respectively. A Birch-Murnaghan fit to the katoite data gives K0=66¿4 GPa and K'0=4.1¿0.5. For grossular we determined K0=168¿25 GPa with K'0=6.2¿4. A better constrained fit is obtained K0=168¿25 GPa with K'0=6.2¿4. A better constrained fit is obtained using K0=168.the present data to give K'0=6.1¿1.5. The low bulk modu attributed largely to the highly compressible tetrahedral O-O distances in the structure. ¿American Geophysical Union 1991