Serine monohydrate undergoes a phase transition at 5.2 GPa which involves reorientation of water molecules. It appears that this disrupts H-bonding elsewhere in the crystal, leaving two strong H-bond donors to make only 'poor' H-bonds. This time is requested with the aim of determining the positions of the H-atoms in the structure. Previous work on PEARL using the Paris-Edinburgh cell indicates that it is an ideal instrument for the study of small molecular systems at pressures less than 10 GPa. A single run from ambient pressure up to 8 GPa in 1 GPa steps with a Pb pressure-marker and MeOD/EtOD as a hydrostatic medium will provide data suitable for analysis of the H-atom positions, and confirm the phase behaviour beyond the regime sampled in the single crystal experiments. We anticipate the need for 4 days of beamtime for this study.