High pressure X-ray diffraction experiments of hydrous ringwoodite, containing 2.8¿0.2 wt% H2O, have been carried out by using X-rays from a synchrotron radiation source with a diamond anvil cell under hydrostatic pressure up to 6 GPa. The isothermal bulk modulus (K0) was determined to be 148¿1 GPa with the pressure derivative (K0') fixed to 5 using the Birch--Murnaghan equation of state. This value is almost 80% of that of anhydrous ringwoodite. The relations of the bulk modulus and zero pressure volumes (V0) with H2O content are estimated within the solid solution range. Combining the present results with recent seismological data, the nature of the 520 km and 660 km seismic discontinuities is discussed. ¿ 2000 American Geophysical Union