A mullite precursor solution was made by hydrolyzing TEOS in a diluted aqueous solution of aluminum nitrate. This solution was spray‐dried at a relatively low temperature (200°C), producing a chemically homogeneous powder crystallizing completely into mullite at 975°C. By aging this solution at 100°C, a diphasic precursor sol made of colloidal silica and aluminum nitrate was made. The resulting spraydried powder was less homogeneous, crystallizing into mullite and spinel or only spinel. The aging by thermal treatment may thus be a way to control the heterogeneity of the po