By adsorption of monosilicic acid on aluminum hydroxide, an amorphous aluminosilicate (AAS) of Si͞Al ratio of 1͞3 was formed. When the AAS was heated to 1000 ± C, mullite (3Al 2 O 3 ? 2SiO 2 ) crystallized directly without the formation of a spinel phase. It was clearly evidenced that correct adjustment of three factors, the Si͞Al ratio, the proportion of 4-and 6-coordinated aluminums, and the single environment of a SiO 42 4 unit (no Si-O -Si bond), is important for the direct mullitization from AAS.