In-situ high pressure X-ray diffraction of phase E (free of secondary phases) measured up to 14.5 GPa shows that phase E is stable over this pressure range at room temperature. The pressure dependence of the lattice parameters are a = 2.967 − 0.011P + 0.0001P2 and c = 13.886 − 0.054P + 0.001P2 (P is in GPa). A least-squares fit to third-order of Eulerian strain theory yields a bulk modulus KT0 for phase E of 93 (±4) GPa and pressure derivative K′T0 of 5 (±1). The bulk modulus obtained by this study is about 10% lower than that obtained by Brillouin scattering. Phase E appears to have the lowest bulk modulus among DMHS. The OH stretching frequency of 3613 cm−1 indicates weak, if any, hydrogen bonding. The associated O-O distance of phase E is estimated to be 3.00–3.10 Å.