Silica and stannic (tin) oxide powders were synthesized by oxidation of their respective chlorides in single and double (inverse) diffusion flame reactors. The effect of reactant gas mixing on the characteristics of these powders (size and morphology) was investigated by altering the position of the fuel (CH4) and oxidant (air or O-2) streams in the burner Reactant gas mixing plays a key role in controlling particle size since it affects the temperature history, residence time, and initial particle concentration in the flame, thus, yielding a simple technique for particle-size control over a wide size range in flame reactors. The different material properties (such as sintering) of silica and stannic oxide result in particles of different size and morphology, although they were made at nearly identical flame conditions. Moreover, the oxidant composition affects significantly the properties of silica particles.