Dynamic TG investigations were carried out to elucidate the mechanisur of thermal decomposition of aluminium sodium sulphate crystallohydrate. Shimadzu 31H and MOM Derivatograph X-ray diffraction and other techniques were used to determine data on the decomposition, activation energy, structure and phases in the solid products. Isothermal study in the temperature ranges 883-958 K and 983-1113 K in air or a reducing gas atmosphere revealed different reaction mechanisms. Depending on the experimental conditions, mainly Al2O3 can be obtained.