The scale-up and design of flame aerosol reactors are investigated for synthesis of silica and titania nanoparticles. In specific, coflow burners of different dimensions are studied at various precursor, fuel, and oxygen flow rates. The influence of the flame enthalpy content on product primary particle size is investigated by changing the fuel from methane to propane or hydrogen. Operation lines relating product particle size with reactant outlet conditions, burner size, and flame enthalpy are developed, showing how the different reactors can produce silica or titania nanoparticles of the same size. A scale-up procedure developed for fumed silica is extended to the synthesis of titania nanoparticles covering production rates of 2-200 g/h. At high fuel-oxidant velocity difference at the burner outlet, the operation of diffusion flame reactors converges to that of premixed ones. (c) 2004 Elsevier B.V. All rights reserved.