Silver nanoparticles were produced in the Rapid Expansion of a Supercritical Solution into a Liquid SOLVent (RESOLV). Perfluorinated surfactant-stabilized water-in-CO2 microemulsion was used to dissolve silver salt for the rapid expansion. It was found that well-behaved nanocrystalline silver particles could be obtained when the reductive solution at the receiving end of the rapid expansion was adjusted to be highly basic. The results suggest that the pre-expansion reverse micelles in the microemulsion might be playing a significant role in the formation of the nanoparticles and that such a role could be affected by the basicity of the receiving solution in RESOLV. The apparent correlation between the parameters of the pre-expansion microemulsion and the properties of the produced nanoparticles is discussed. (c) 2004 Elsevier B.V. All rights reserved.