We described the possible roles of an inert salt such as NaCl in spray pyrolysis and aerosol-gel syntheses of nanoparticles. We demonstrated that the salt acts primarily as a filler to inhibit further agglomeration of preexisting SiO2 sols as well as co-precipitated CuO particles. Depending on the contents of the salt, we observed that the microstructures of nanoparticles range from nano dots to mesoporous particles. During the aerosol-gel reaction, in particular, the salt could accelerate the reaction faster. Possible mechanisms will be discussed in detail. (C) 2006 Elsevier B.V. All rights reserved.