With the recent interest in the chemical, electronic and optical properties of nanometer scale metal particles, there is now interest in manufacturing these materials in larger quantities. Since both small particle size and high particle number concentrations are sought, there is a need for improved. particle generation reactors that can realize both goals. Here, results are presented for the synthesis of indium metal nanoparticles in an evaporation/condensation aerosol generator. Size distributions were measured for metal nanoparticles formed using a standard flow configuration, as well as using several variations on the standard configuration. The aim of the modifications is to increase the cooling rate and thus, to increase the nucleation rate of the nanoparticles. An increase in the number concentration of particles and, in some cases, a significant decrease in average particle size was observed when the modified reactor configurations were used. These results can be explained by the changes in the time-temperature history of the nanoparticles resulting from the modifications to the aerosol generator. A monodisperse model of nanoparticle formation and growth, accounting for nucleation, condensation and coagulation, was used to describe particle formation in the standard flow configuration, to guide the modifications, and to describe particle formation in one of the modified configurations, with qualitative agreement seen between measured and predicted particle sizes. (C) 2002 Elsevier Science Ltd. All rights reserved.