The aim of the experiment is to use neutron powder diffraction to determine the crystallographic site occupancies for a number of half-Heusler compounds. These materials are of interest for use in thermoelectric waste heat recovery. Accurate knowledge of the unit cell contents, in particular the the distribution of the metals present over the available crystallographic sites is essential to understand the measured thermoelectric properties and will enable further targeted improvements to be made. The compositions that we would like to investigate include half-Heuslers based on TiNiSn, NbCoSn and VFeSb. Neutrons are needed because of the strong scattering contrast between the first row transition metals.