Phase B synthesized from a natural peridotite has been studied to 30.5 (¿1.5) GPa on compression and decompression using infrared spectroscopy. The mode shifts of the OH1 (3345 cm-1) and the OH2 (3404 cm-1) vibrations of phase B are extremely small, -0.8 cm-1/GPa and 0.9 cm-1/GPa, respectively, indicating that the hydrogen bonding in this phase is extremely insensitive to pressure. The opposite signs of the two mode shifts indicate that the OH1 hydrogen bond becomes stronger on compression, while the OH2 hydrogen bond weakens. Because both hydroxyl units are hydrogen bound to the same oxygen, this behavior may reflect hydrogen-hydrogen repulsion in these sites as the lattice undergoes pressure induced compaction. Our results indicate that the environment surrounding the hydroxyl units in this phase is significantly more rigid than that present in a range of other hydrous magnesian silicate phases. Systematics of vibrational spectra among hydrous phases in the MgO-SiO2-H2O system indicate that pressure enhances hydrogen bonding through polymorphic transitions; yet, as illustrated by our results, the effect of pressure on hydrogen bonding within any single phase is small. Therefore, variations in hydrogen bonding may play a primary role in the notable polymorphism of hydrous phases at high pressures. ¿ American Geophysical Union 1996