Chemical compositions and crystal structures of coexisting wadsleyite, clinopyroxene, and phase E (synthesized at 1400 °C and 18 GPa under hydrous conditions) have been analyzed by electron microprobe, single-crystal X-ray diffraction, and Raman spectroscopy. Single-crystal X-ray diffraction analyses indicate that Fe2+ substitution in the crystal structure significantly decreases the configurational entropies of wadsleyite and clinopyroxene in the middle transition zone under water-saturated conditions, providing a new thermodynamic constraint on the stability of ferromagnesian silicates near the wadsleyite–ringwoodite phase boundary (about 520 km). The variation in the depth of the 520-km discontinuity under hydrous conditions can be partly due to Fe2+ incorporation into wadsleyite.