In this articles, we present the formation of nanosized metallic copper particles synthesized in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles in compressed propane and supercritical fluid (SCF) ethane solutions. Specifically, copper particles (<20 nm) were produced by the reduction of copper ions from copper bis(2-ethylhexyl) sulfosuccinate, Cu(AOT)(2), incorporated within AOT reverse micelles dispersed in compressed propane and SCF ethane solvent along with small amounts of isooctane cosolvent. The effects of the compressed solvents on the particle characteristics and particle growth rate are discussed and comparisons are made with growth rates in normal liquid solvents. The metallic nanoparticles were characterized using both in situ UV-vis spectroscopy and transmission electron microscopy. The UV-vis spectrum of these nanosized copper particles is sensitive to particle size, and hence time-resolved spectral measurements were used to monitor the particle growth process. The particle growth rate in AOT reverse micelles in SCF ethane is faster than in either of the two normal liquid solvents partly because of intermediate physical and transport properties of the SCF solvent (e.g. increased mass transfer, decreased viscosity and density, lower dielectric constant), which results in increased collision frequencies and exchange rates between micelles.