Diphasic monolithic mullite gel of stoichiometric composition shows a multiple aggregation of colloidal SiO2 (similar to 12 nm) and AIOOH (similar to 10 nm) particles. The identity of the colloidal particles is retained up to the crystallization of mullite at a temperature of 1240 degrees C in unpressed or 1220 degrees C in gels cold-isostatically pressed at 1.5 GPa. Intensive sintering is closely followed by crystallization of mullite. Both sintering and crystallization are apparently related with the formation of a silica continuum. Nucleation of mullite appears to occur at points of contact (interface) of SiO2/delta-Al2O3 particles.