This paper presents the formation of nanosized metallic particles synthesized in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles in compressed propane and supercritical fluid (SCF) ethane solutions. Silver nanoparticles were produced in AOT reverse micelles in compressed propane by the reduction of the silver salt, AgNO₃. Copper nanoparticles were produced in compressed propane and SCF ethane solvents, with added isooctane cosolvent, by the reduction of the functionalized surfactant, Cu(AOT)₂. Particle growth rates in the compressed fluids are compared with those in normal liquid solvents to illustrate the effects of the compressed fluids on the growth behavior of these nanoparticles within the reverse micelles. The metallic nanoparticles were characterized using both in situ UV−vis spectroscopy and transmission electron microscopy. The use of compressed fluids and SCFs for nanoparticle formation in AOT reverse micelles can result in growth rate behavior different from that in normal liquid solvents for both copper and silver. This is the result of the intermediate physical and transport properties of the compressed and supercritical solvents which lead to enhanced collision frequencies and exchange rates between micelles.