The thermal behavior of Al₂O₃/NH₄HF₂ mixtures with different mass ratios of NH₄HF₂:Al₂O₃ were analyzed by simultaneous thermogravimetry and differential thermal analysis (TG-DTA) and the critical temperature of DTA curve were determined. The morphologies and phases of the products obtained by direct thermal treatment before and after each critical temperature were further analyzed. The results show that the mass ratio has no influence on the critical reaction temperature and processes. The fluorination starts at room temperature with the formation of (NH₄)₃AlF₆, which dominates at 162.3~162.8 °C and is completed around 180 °C. After further heat-treatment, (NH₄)₃AlF₆ decomposes sequentially through a two-step decomposition reac-tion with the formation of NH₄AlF₄ at 249.8~250.1 °C and finally decomposes to β-AlF₃ at 356.8~357.7 °C. The transformation of β-AlF₃ to α-AlF₃ occurs at 400~650 °C.