The thermal decomposition of aluminum fluoride trihydrate was investigated by TG-DTG. The process was recognized to involve three stages: I AlF3 . 3H(2)O(108-227 degrees C) --> AlF3 . 0.5H(2)O + 2.5H(2)O II AlF3 . 0.5H(2)O(277-550 degrees C) --> AlF3 + 0.5H(2)O III AlF3. 3H(2)O(>380 degrees C) --> Al2O3 + 6HF It was found that the apparent activation energy, E, and the preexponential factor, A, for stage I reaction were 41.9 kJ mol(-1) and 10(2.49) s(-1), respectively. For stage II reaction, the value of E was 47.5 kJ mol(-1) and the most probable differential mechanism function was in the form of (1-alpha), and magnitude of 80.4 kJ mol(-1) of E and a function in the form of 3/2(1-alpha)[-ln(1-alpha)(1/3)] were derived fur stage III reaction, From the measurements conducted under increasing temperature conditions, contents of fluorine, C-F(%), on moisture-free basis and water, C-W(%), on current mass basis contained in the AlF3. 3H(2)O sample were determined and correlation between the temperature range dictated by the quality specification of AlF3, C(F)greater than or equal to 61.0%, C-W<0.5%, and the heating rate were presented. The content of fluorine and water were also determined under isothermal conditions and correlation between the time duration of the treating temperature necessary for the quality specification to be met, and the magnitude of temperature were presented.