The effect of H2O on the high-pressure phase transformations in a 1:1 mixture of forsterite and enstatite has been investigated on the basis of experimental data available in the MgO-SiO2-H2O system at pressures up to 280 kbar. It has been found that the modified spinel phase of Mg2SiO4 does not occur at high pressure in this system, where more than about wt % H2O is present. In addition, the spinel phase of Mg2SiO4 does not appear at its equilibrium transition boundary (modified spinel⇄spinel) if more than about 3 wt % H2O is present in the system. Thus, if more than about 4 wt % H2O is present in the chemical composition of downgoing slabs, the so-called 400 km seismic discontinuity in the normal mantle may not occur in the subduction zone at all.