The formation of mullite was investigated using microcomposite powders which consist of α‐alumina cores and amorphous silica coatings. Differential thermal analysis and X‐ray diffraction showed that the mullitization reaction was endothermic. In contrast, mullite forms exothermically in samples prepared by sol‐gel processing. The results are shown to be consistent with available thermodynamic data for mullite formation from different alumina and silica