The structure of mullite was refined by the Rietveld method, and a new method for the estimation of chemical composition of mullite was proposed. Mullites were synthesized by firing the starting materials prepared by sol–gel and spray pyrolysis methods at various temperatures. Linear correlations were recongnized between the chemical composition of mullite analyzed with the Rietveld method and the lengths ofaandbaxes, but a small anomaly was found in the variation of thecaxis. It was found to be caused by a difference in thecaxis of pseudotetragonal mullite and that of orthorhombic mullite. A good correlation was found between the chemical composition of mullite and the intensity ratio of (I220/I111). This is a much more convenient method for estimating the chemical composition of mullite than Cameron's method, which uses a lattice constan