Mechanical mixture of gamma-Al2O3 and amorphous SiO2, and diphasic Al2O3/SiO2 gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900degrees-1600degreesC. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of theta-Al2O3 and alpha-Al2O3 polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al-Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.