We propose to investigate the changes that occur in the structure of ammonium formate as a function of pressure. This is the simplest system in which the oxygen atoms of a carboxylate group are hydrogen bonded to the hydrogen atoms of an NH group. This compound is known to undergo a phase transition at 1.8 GPa, during which there is a significant reorganization of the hydrogen-bonding network. We will determine the evolution of the changes within the hydrogen bonds for both phases. In particular the hydrogen bonding in the high pressure phase is such that for on the oxygen atoms the intermolecular bonding occurs within the plane of the carboxylate group whilst for the other oxygen it is at right angles to this plane. This bonding arrangement gives us an opportunity to investigate the effect of pressure on a system where both in and out of plane hydrogen bonding occurs.