Abstract The primary and secondary mullite formation in high purity kaolinite-α-alumina have been studied by conventional processing, XRD, TEM, AEM, and optical microscopy. After 4 h firing at 1300°C or below, only monosized primary mullite was formed in platy kaolinite, leaving α-alumina largely inert. Bimodal mullite crystals demonstrating secondary mullite nucleation and priamry mullite growth were observed after 4 h firing at 1400°C. At 1400°C and above, the rapid kinetics of secondary mullite formation, the characteristics of the co-existing glassy phase and the microstructure, as well as the evidence of liquid phase sintering strongly that solution-precipitation via a transitory liquid phase was the major mechanism for secondary mullite formation. Secondary mullite formation via inter-diffusion, though also observable, was considered as a minor route in the kaolinite-α-alumina reaction.