Hybrid gels of celsian composition were prepared from Al alkoxide, tetrathylorthosilicate (TEOS) and Ba acetate and their structure evolution was studied up to 1300 degrees C by thermal analysis and X-ray diffraction. Information on their precrystallization behaviour was also provided by Al-27, Si-29 and Ba-137 MAS NMR spectroscopy. Apart from some excess Ba acetate which decomposed to traces of BaCO3 and BaO by ca. 500 degrees C, the gels are X-ray amorphous and relatively homogeneous, and begin to crystallize to hexagonal celsian at 900 degrees C. From approximate to 500 degrees C onwards, an Al-substituted tetrahedral SiO4 framework begins to be established, evidenced by a progressive increase in the tetrahedral Al-27 sites and the Q(4)(4Al)Si-29 resonance. Migration of Ba into the polyhedral celsian sites occurs much more slowly. A small amount of mullite and Ba2SiO4 which crystallize from Al-rich and Ba-rich regions, respectively, also form crystalline celsian in secondary reactions at ca. 1100 degrees C, The observation of a Al-27 shoulder at ca. 36 ppm at 500-900 degrees C may arise from Ba-poor mullite-like regions.