The thermal behavior of a series of Ga-substituted hydrotalcites [Mg1−xGax(OH)2] (CO3)x/2·mH2O, where x = 0.072−0.35, was examined by means of thermal gravimetric analysis, (TGA), differential thermal analysis (DTA) and x-ray diffraction (XRD) measurements. The weight loss patterns of Ga-hydrotalcites depend on the Mg/Ga molar ratios. For example, a high [CO3]2− density in the interlayer region of Ga-rich samples (i.e. Mg/Ga = 1.8) makes them more difficult to dehydrate and slower to decompose than Ga-poor samples (i.e. Mg/Ga = 12.9). Removal of interlamellar water molecules in Ga-rich hydrotalcites consumes less energy than that in Ga-poor ones. The decomposition stage, regardless of Mg/Ga ratios, takes place at a constant temperature of ≈ 410 K, but this process requires much less energy in Ga-rich (Mg/Ga = 1.8; 22 J g−1) than in Ga-poor samples (Mg/Ga = 12.9; 202 J g−1). XRD analysis indicates that the layered structure is destroyed below 573 K, and that between 623 and 973 K, Ga-hydrotalcites convert into MgO-like materials, probably substituted by Ga3+. Calcination temperatures above 1073 K yield MgO-MgGa2O4 mixtures. © 1997 Elsevier Science Ltd