Ultra-fine mullite precursor powders were prepared by the hydrolysis of molecular-designed Al.Si-double alkoxide. The surface areas of the resulting mullite powders derived from the sol-gel route varied depending upon the aging and/or drying condition, due to the different agglomeration states of the primary particles, precipitated by the hydrolysis of the precursor solutions. In the case of the solvents with low dielectric constants, the thin electrical double layer is formed on the primary particles and then primary particles agglomerated into coarse secondary agglomerates, leading to mullite powders with high surface areas even after high-temperature calcination. A thick electrical double layer on the primary particles resulted in uniform packing, which formed relatively dense secondary agglomerates during drying, leading to lower surface areas, when solvents with high dielectric constants were used during aging. This agglomeration behavior was confirmed by the pore size distributions and SEM observation of the mullite powders.