The flame spray pyrolysis (FSP) process was systematically investigated using an external-mixing gas-assisted atomizer supported by six premixed methane-oxygen flameletes. The effect of oxidant and precursor fuel composition on the size of FSP-made silica primary particles (8-40 nm) was studied using as precursor hexamethyldisiloxane (HMDSO) dissolved in ethanol, iso-octane or methanol. As oxidant air and pure oxygen were used, that served also as droplet dispersion gases. Droplet size distributions were measured by laser diffraction, while droplet lifetimes were calculated using a spray combustion model to explain for the first time the difference in flame structure and especially product powder characteristics when air or oxygen was used as oxidant/dispersion gas. The spray flame temperature was measured by Fourier transform infrared (FTIR) emission/transmission (E/T) spectroscopy. The liquid solvent (fuel), especially its enthalpy content, was an important parameter in FSP as it affected the total net heating value of the spray flame. It is shown for the first time also how the specific surface area of the FSP-made particles can be systematically controlled through the oxidant flow rate and precursor/fuel composition.