NiSi is a technologically important material that is also of interest to Earth Scientists because of the possible role of Ni and Si in the Earth¿s core. It crystallises in the orthorhombic MnP structure and shows unusual behaviour when heated above room temperature (termed "enormous anisotropy"), with a strong contraction of the b-axis. No crystallographic studies of NiSi below room temperature have been reported. We wish to investigate the behaviour of this unusual material as a function of temperature by using HRPD to determine its lattice parameters and atomic coordinates from 4 to 1225 K.