Water/supercritical CO2 microemulsions were prepared by using ammonium carboxylate perfluoropolyether (PFPE) as a surfactant. The de-aggregation of the J-aggregate of cyanine dyes and the formation of fairly small silver particles in these microemulsions would indicate the realization of the chemical reactions in the small space. In this sense, the water/supercritical microemulsion is regarded as a novel "microreactor" of the nanometer size.