Ba0.75Sr0.25AlSi2O8 (SBAS) - Al2O3 composites, with SBAS/Al2O3 weight ratios of: (a) 90/10, (b) 70/30, and (c) 50/50, are in situ synthesized by reactive sintering at 900-1500°C/5h. The effect of mechanical activation of the precursor mixtures for 0, 4 or 8h in an attrition milling device on the microstructure and phase composition of the composites is studied. Only SBAS and Al2O3 phases are obtained at 1300-1500°C, independently of milling time. In general, the relative proportion of the desirable monoclinic SBAS (Celsian) phase increases in the materials with increasing milling time and sintering temperature, which is enhanced by their SrO content. The promotion of surface nucleation of the undesirable hexagonal SBAS (Hexacelsian) phase by mechanical activation results in a maximum Hexacelsian to Celsian conversion fraction of only 81.4%, obtained for composition 2 milled for 8h and sintered at 1500°C/5h. Under these synthesis conditions, an increment in the amount and size of the Al2O3 particles in the composites is detrimental for the Hexacelsian to Celsian conversion.