Silver nanoparticles were prepared by using polyvinyl pyrrolidone (PVP) as a stabilizer and gamma-irradiation. Transmission electron mi-croscopy (TEM) results showed that both the amount and the molecular weight of PVP in the irradiated solution considerably affect the average size of the silver nanoparticles. The average size of the silver nanoparticles decreases with increasing the amount of PVP in the solution, but increases with increasing its molecular weight. Further, TEM showed that the silver nanoparticles become disassembled into smaller nanoparticles after dilution with distilled water and sonication. Since the processes of dilution and sonication are not expected to result in chemical reactions or to split the silver nanoparticles, we conclude that each silver nanoparticle prepared by gamma-irradiation consists of several smaller nanoparticles surrounded by PVP. Thus, based on these observations, we propose a three-step mechanism for the growth of the silver nanoparticles under the conditions considered here. In the first step, the silver ions interact with PVP, then in the second step the silver ions that are exposed to gamma-irradiation are reduced to silver atoms; nearby silver atoms then aggregate at close range. These aggregates are the primary nanoparticles. Finally, these primary nanoparticles coalesce with other nearby primary nanoparticles or interact with PVP to form larger aggregates which are the secondary (final) nanoparticles. (C) 2004 Elsevier Inc. All rights reserved.