Abstract Silica particle formation was studied in a counterflow diffusion flame, under different operating conditions, in order to investigate the effect of process variables on particle formation. The counterflow geometry provides a one-dimensional flow field, along the stagnation streamline, which greatly simplifies interpretation of the particle formation process. Silane has been used as the source of silicon in hydrogen/oxygen/argon flames. Silica particle characteristics (size, number density, volume fraction) have been determined using dynamic light scattering and angular dissymmetry measurements. The effects of silane loading, temperature, equivalence ratio, and momentum ratio have been investigated. The results indicate that temperature plays a significant role in the formation process, by controlling the chemical kinetic rates and through physical changes in particle morphology. In cooler flames ( T ≈ 2000 K), significantly more surface growth is observed. Increasing flame temperatures tend to enhance homogeneous nucleation, leading to smaller particles in higher numbers. Results obtained in higher-temperature flames ( T ≈ 2500 K) suggest that, when the chemical kinetic rates increase substantially, particles of composition other than SiO 2 can also nucleate, which may then grow by silica deposition. It was also observed that, under conditions where particles were hot enough, sintering effects produced more spherical structures. The level of silane loading is seen to affect the final particle size obtained as well as the growth regimes that particles encounter prior to ejection from the reactor. Larger silane loadings tend to accelerate particle nucleation, form larger particles, and enhance surface growth effects. Finally, the reactor provides a synthesis environment in which the process parameters in a flame can be varied in order to significantly alter particle morphology and end product yield.