SiO2, Al2O3, and 3Al2O3.2SiO2powders were synthesized by combustion of SiCl4or/and AlCl3using a counterflow diffusion flame. The SiO2and Al2O3powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al2O3.2SiO2powder produced with a low‐temperature flame was also amorphous and had a similar morphology. However, those produced with high‐temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the fl