The phase boundary between ringwoodite and perovskite plus periclase in Mg2SiO4 was determined at 1600 ¿C under hydrous conditions with 1~3 wt% H2O. We found that the boundary moved to high-pressure side by ~0.2 GPa in these hydrous conditions compared with that in anhydrous condition. The SIMS measurements confirmed that the ringwoodite in hydrous condition contained a significant amount (~1 wt%) of water, whereas the perovskite accepted only very little (~0.05 wt%) water in its crystal structure. The discrepancy between the pressure of 660 km seismic discontinuity and that of the postspinel transformation reported in a recent in situ x-ray study is not explained by the effect of water on the transformation. ¿ 2001 American Geophysical Union