NaLa(SO4)2,H2O crystalline powder was obtained under hydrothermal conditions at 220°C. A coupled TGA/DTA experiment of NaLa(SO4)2,H2O exhibits a weight loss at 260°C corresponding to the dehydration and an endothermal peak at 774°C. To elucidate the transformation mechanism as a function of temperature, single crystals have been grown at 80°C, 300 and 800°C. For each phase, single crystals have been isolated and structure determination was performed. As already published, NaLa(SO4)2,H2O crystallizes in a P3121 space group. However, the dehydration at 260°C is not a simple loss of the water molecule but a radical change in the structure. The removal of the water molecules inside the tunnels formed by the framework leads to a change in the coordination of the LaO9 Lanthanum-based polyhedrons. The compound obtained after dehydration is a new triple sulfate of the formula Na3La(SO4)3 crystallizing in the R-3 space group (a ​= ​14.0976(1) Å; c ​= ​8.1267(1) Å) with LaO12 icosahedrons. Millimeter size single crystals of this new phase have been grown under hydrothermal conditions (300°C, 157 ​bars). After the endothermal peak at 774°C, Na3La(SO4)3 decomposes by forming the anhydrous double sulfate NaLa(SO4)2 crystallizing in the P-1 space group with LaO10 polyhedrons. The structure of the three (NaLa)-compounds at RT, 300°C and 800°C is compatible with the expected Raman signatures. Finally, a complete transformation of NaLa(SO4)2,H2O up to 800°C is proposed. After 1000°C, the compound decomposes chemically with a large weight loss.