The scaling-up of diffusion flame aerosol reactors is investigated for synthesis of nanoparticles. Three co-flow burners of different dimensions are studied at various precursor, oxidant and fuel flow rates. An operation line relating product particle size with reactant outlet conditions, flow rate and burner size is developed showing how the three reactors can produce silica and titania nanoparticles of the same size and morphology. This operation line shows the limit of fast reactant mixing where the diffusion flame aerosol reactors perform as premixed ones. An operation diagram is obtained for different silica production rates and a scale-up procedure is developed. (C) 2003 Elsevier Ltd. All rights reserved.