Neutron powder diffraction studies of 154Sm58NiO3, 154Sm60NiO3, and 154Sm62NiO3, at a range of temperatures through the M−I transition at approximately 128 °C, have been performed on the new general materials diffractometer (GEM) at ISIS, RAL. With combined data-set Rietveld analysis, using samples containing different nickel isotopes with contrasting scattering lengths, it has been found that extremely high quality structural parameters can be determined, even though total data collection times are more than an order of magnitude shorter than those previously used for this system. Rietveld analysis shows that the evolution of the structural parameters over the temperature range are smooth and that no symmetry change or abrupt structural transition occurs at the M−I transition. This is consistent with evolution of the high-temperature metallic material within the low-temperature insulating phase over the temperature range 108−128 °C. The key effects of thermal motion on the M−I transition have been extracted from the data and are discussed.