The high temperature transformation of sillimanite to mullite has been studied by means of dilatometry, X-ray diffraction, FT-IR and scanning electron microscopy (SEM-EDS) techniques. The results obtained showed that total transformation of sillimanite to mullite takes place between 1550 and 1600 o C. A rapid nucleation occurs above 1550 o C due to the resemblance of sillimanite and mullite structures. As a consequence of the high temperature of mullitization of sillimanite, the grain growth of elongated mullite crystals takes place almost simultaneously with the nucleation stage. Finally, a step of acid leaching was employed to remove the glassy phase and high-grade purity mullite powders were obtained.