On heating, hydrated boron minerals, colemanite and ulexite lose their water of crystalisation while undergoing various mineralogical and structural changes. In this study, the nature and mechanisms of the thermal reactions of ulexite have been determined by TG, DTG and DTA. The changes in the structure have been investigated by XRD, SEM, and Hg-porosimetry. Thermal decomposition of ulexite occurred within the temperature range 60-500 degreesC with two dehydration stages proceeded by two dehydroxylation stages. The structure was first transformed into NaCaB5O6(OH)(6). 3H(2)O within 60-180 degreesC, then into NaCaB5O6(OH)(6).H2O within 151-260 degreesC. Amorphisation took place along with dehydroxylation. The structure was first transformed into a multi-domain heterogeneous matrix consisting of NaCaB5O6(OH)(6).H2O and an amorphous phase to X-ray during the first stage of dehydroxylation within 180-260 degreesC. Then remaining hydroxyl groups removed between 260 and 500 degreesC. The sample has become completely X-ray amorphous. During the gradual liberation of water vapour, the structure developed numerous microcracks and had exfoliated without decrepitating. The microcracks increased the degree of porosity and the structure became more easy to grind. Furthermore, the amorphous structure recrystallised as NaCaB5O9 at 636 degreesC and transformed into CaB2O4. Any remaining NaB3O5 after 855 degreesC was amorphous.