The effect of varying the aluminium oxide content on the nucleation and crystallization behaviour of barium containing glasses based on 8SiO(2).YAl(2)O(3)4MgO2MgF(2)BaO was investigated in order to develop novel, high strength, machinable glass-ceramics. Nine glasses were synthesized and characterized by Differential Scanning Calorimetry (DSC) Combined Differential Thermal Analysis Thermal Gravimetric Analysis (DTA/TGA), X-ray diffraction (XRD) and dilatometry.The glass transition temperature (T-g) and first peak crystallization temperature (Tp1) reduced with reducing alumina content. Glasses with Y > 1.5 exhibited a second peak crystallization temperature (Tp2). Tp1 was shown to correspond to the crystallization of barium fluorphlogopite (BaSi6Al2Mg6F2O19) and Tp2 to the crystallization of cordierite ( Mg2Al2Si5O18). The thermal expansion coefficient (TEC) was insensitive to alumina content. All the glasses exhibited an optimum nucleation temperature just above T-g, which was thought to be a result of amorphous phase separation (APS). DTA/TGA showed the glasses to undergo weight loss corresponding to silicon tetrafluoride volatilization from the surface, which resulted in a fluorphlogopite deficient surface layer. (C) 2004 Kluwer Academic Publishers.