This work addresses the formation conditions and stability of a celsian-cordierite eutectic structure produced during the crystallization of a barium magnesium aluminosilicate (BMAS) glass-ceramic with the Ba-osumilite stoichiometry. The melt-derived BMAS glass-ceramic was pulverized and subsequently hot pressed to produce sintered powder compacts, some of which were subjected to post-sintering heat treatments in air. The study of the devitrified BMAS glass revealed that a metastable celsian-cordierite eutectic structure resulted from the prolonged annealing of the BMAS glass-ceramic between 860 degrees and 1000 degrees C. The celsian-cordierite eutectic structure transformed to the thermodynamically stable Ba-osumilite phase when annealed at temperatures > 1000 degrees C.