We will investigate the structure of 4-chloroimidazole a material which undergoes extreme anisotropic thermal expansion. This is believed from our initial X-ray data to be due to a balance between weak C-H...Cl bonds and strong N-H...H hydrogen bonds. Data collected on HRPD will be used to correlate the changes at the atomic level in the strong N-H...N bonds and weak C-H...Cl bonds, with the macroscopic changes in the lattice parameters, (expansion coefficients), by carrying out a combination of accurate structural refinements coupled a large number of lattice parameter measurements. In addition to the fundamental interest in the properties of hydrogen bonded systems the information gained from this experiment will be desirable in designing functional materials, as the aforementioned properties may be used to tune the response of a thermally-driven actuator or switch.