The thermal decomposition of a synthetic hydrotalcite-type (HT) sample [Mg0.71Al0.29(OH)(2)(Co-3)(0.145). mH(2)O] has been investigated by neutron diffraction using a programmed temperature control facility. The evolution from HT phase to mixed oxide leading ultimately to the partial segregation of a spinel phase is reported. Structural modification ascribed to the interlayer water loss from the HT phase is accompanied by a reduction of the c-axis parameter and changes in the neutron powder pattern. The neutron diffraction patterns of five Mg/Ni/Al mixed oxides obtained from FIT samples by calcination at 923 K were subjected to structural refinement by the Rietveld method. The model that best describes the refined data consists of a supercell with an a-axis twice that of the stoichiometric rock-salt type oxide, a certain degree of ordered cation vacancies in the octahedral sites, and partially occupied tetrahedral sites. Although the samples essentially show the typical patterns of a rock-salt type structure, the results indicate that they conform to a spinel type structure, which corroborates the deductions made by workers using other techniques. A high replacement of aluminium for magnesium in the magnesium-richer samples is proposed.