Solid-state ²⁷Al and ²⁵Mg MAS NMR and X-ray powder diffraction in combination with thermal analysis indicates that synthetic hydrotalcite decomposes thermally in at least three weight-loss steps. The first step, at 250–280 °C, principally involves the loss of the interlayer water, but the appearance of some tetrahedral Al in the ²⁷Al NMR spectra suggests that this is accompanied by the onset of Al dehydroxylation. The second step, at 300–400 °C, is accompanied by a rapid increase in the proportion of tetrahedral Al to a maximum at 400 °C, indicating full Al dehydroxylation of the Al regions. The third step, above 400 °C, represents loss of interlayer carbonate and dehydroxylation of Mg and is accompanied by the appearance of poorly crystalline MgO in both the X-ray and ²⁵Mg NMR spectra. In the range ca. 400–900 °C, we suggest that the phase assemblage consists of intergrown microdomains of poorly crystalline MgO, possibly containing substituent Al³⁺ and vacancies, and a phase with NMR characteristics similar to spinel-based aluminas such as γ-Al₂O₃. By 1200 °C, migration of Al from the MgO phase and of Mg into the spinel has occurred, resulting in the appearance of crystalline pure MgO and MgAl₂O₄ with an inversion parameter of ca. 0.3.