Formation of particles in a H2-O 2 counterflow diffusion flame doped with Sill 4 or SiCI 4 was investigated. When Sill 4 is used as precursor, the species that nucleate in the flame depend on the Sill 4 concentration as well as on the flame temperature. At low Sill 4 concentrations and/or when a high-temperature flame is used, SiO 2 is the species that nucleates. At high Sill 4 concentrations and/or when a low-temperature flame is used, nucleation of SiO x (x = 0 and/or 1) in addition to SiO2 is believed to occur in the flame. When SiCI 4 is used as precursor, SiO2 is the species that nucleates under all the experimental conditions studied here. Laser light scattering and extinction measurements indicate a specific region in the flame where growth of the particles by coagulation occurs, and the particles were always amorphous. When Sill 4 is used as precursor, most of the particles are in the form of agglomerate of small particles (mostly 20-40 nm diameter) with some larger, spherical particles (mostly 80-150 nm diameter). When SiCi 4 is used as precursor, the particles are all agglomerates of small particles (mostly 20-30 nm diameter). When Sill 4 is used as precursor, a second flame, orange in color, appears below the H2-O 2 flame when the Sill 4 concentration is increased beyond a certain value. This orange flame cannot be ascribed to radiation from SiO 2 particles and is suspected to be due to SiO x particles.