The thermal decomposition of metal-substituted ammonium aluminium carbonate hydroxide (AACH) materials with dawsonite-like structure has been studied by using in situ XRD, FT-IR, TGA, and TPD-MS in the temperature range 298–1573 K. The in situ approach enables accurate monitoring of the decomposition process and the nature and stability of the resulting oxide phases. This information is essential to optimize the thermal activation of these versatile materials for subsequent catalytic applications. Pure AACH and the corresponding substituted systems with La and/or transition metals (Fe or Mn) were synthesized by co-precipitation using the carbonate route. In situ XRD indicated the destruction of the AACH phase at 473 K, independent of the composition of the material, leading to an amorphous alumina phase. This transition temperature is in excellent agreement with FT-IR, TGA, and TPD-MS data. The later techniques substantiate the one-step removal of H₂O, NH₃, and CO₂ within a narrow temperature range. In pure AACH, amorphous alumina is transformed into α-Al₂O₃ at 1373 K, while La-containing samples display the hexaaluminate structure above 1423 K. Incorporation of transition metals into the Al and La–Al systems promotes the formation of α-Al₂O₃ and hexaaluminate phases at significantly lower temperatures.