Diphasic mullite gel (Si/Al=1/3) has been synthesizedin.situby hydrolyzing an aqueous solution of silica sol and aluminium salt with the help of NH4OH solution. It has been characterized by DTA. The formation of Si-Al spinel during non-eqilibrium heating of the gel and its subsequent transformation have been recorded by XRD technique. The result shows that Si—Al spinel crystallizes very slowly at 500°-800°C and then very rapidly at ∼ 1200°C which corresponds with the exhibition of a broad exotherm between 1100° and 1300°C in DTA. Accordingly, this exotherm is attributed to the formation of Si—Al spinel. It suddenly transforms to predominant amount of orthorhombic form of mullite with consequent exhibition of an exotherm at 1320°C in DTA. The residual non-crystalline aluminosilicate phase also crystallizes into mullite and accounts for the second mullitization process in diphasic gel.