Perfluoropolyether ammonium carboxylate stabilized water-in-CO2 microemulsion was used to dissolve silver nitrate salt for the RESOLV (Rapid Expansion of a Supercritical Solution into a Liquid SOLVent) process. The solution of reverse micelles with an aqueous silver nitrate core in CO2 was rapidly expanded into a room-temperature solution of sodium borohydride to form silver nanoparticles. The nanoparticles were characterized using optical, X-ray diffraction, and microscopy techniques. The nanoparticle properties were found to be dependent on the pre-expansion microemulsion conditions in CO2.