The transformation of kaolinite to mullite is reviewed. The thermal effects observed during the transformation and the phases which accompany these effects are discussed in the light of the existing concepts. The chemical, thermal, X-ray, electron-optical, infrared, orientational and crystallographic evidences are advanced to reveal the nature of the intermediate phases and to establish the course of the transformation reaction. Strong evidences exist in favour of Al-Si spinel and y-Al203 as the precursor of mullite formation. The formation of y-Al203 phase finds support in the recent ir, r.e.d. and X-ray fluorescence studies whereas the formation of Al-Si spinel phase is necessary to maintain the structural continuity of the phases from kaolinite to mullite as shown by the single crystal X-ray studies. The transformation follows the sequence:(Figure Presented). © 1977, Taylor & Francis Group, LLC.