The thermal stability of [B]ZSM-5 (boronsilicate) has been examined by IR, XRD, ¹¹B MAS NMR and XPS techniques. [B]ZSM-5 and amorphous borosilicate were converted to α-cristobalite at high temperatures(=750℃). However, Na-free amorphous borosilicate was not converted to α-cristobalite at 750℃. Therefore, the presence of Na⁺ ions in [B]ZSM-5 would determine whether this transition occurred or not. When the phase transition to α-cristobalite occurred, most of B atoms tetrahedrally coordinated were released from the ZSM-5 structure and migrated to the exterior surface as a boron compound having trigonal BO₃ units and oxidation states of 3.