  The structural, electronic, and spectroscopic properties of a high-pressure
phase of methane hydrate (MH-III) are studied by first principles electronic
structure calculations. A detailed analysis of the atomic positions suggests
that {\it ionization} of hydrogen-bonded water molecules occurs around 40GPa
and {\it centering} or symmetrization of hydrogen-bonds occurs around 70 GPa.
These pressures are much lower compared with ioninzation around 55 GPa and
centering around 100 GPa in pure ice. The transition may be observed with
low-temperature IR/Raman spectroscopy of OH stretching modes or neutron
diffraction.
