Abstract Hybrid gels of celsian composition were prepared from Al alkoxide, tetrathylorthosilicate (TEOS) and Ba acetate and their structure evolution was studied up to 1300°C by thermal analysis and X-ray diffraction. Information on their pre-crystallization behaviour was also provided by 27Al, 29Si and 137Ba MAS NMR spectroscopy. Apart from some excess Ba acetate which decomposed to traces of BaCO 3 and BaO by ca. 500°C, the gels are X-ray amorphous and relatively homogeneous, and begin to crystallize to hexagonal celsian at 900°C. From ≈500°C onwards, an Al-substituted tetrahedral SiO 4 framework begins to be established, evidenced by a progressive increase in the tetrahedral 27Al sites and the Q 4(4Al) 29Si resonance. Migration of Ba into the polyhedral celsian sites occurs much more slowly. A small amount of mullite and Ba 2SiO 4 which crystallize from Al-rich and Ba-rich regions, respectively, also form crystalline celsian in secondary reactions at ca. 1100°C. The observation of a 27Al shoulder at ca. 36 ppm at 500–900°C may arise from Ba-poor mullite-like regions.