Nanometer-sized silver, copper, and palladium metal particles were synthesized by chemical reduction of Ag+, Cu2+, and Pd2+ ions dissolved in the water core of a water-in-supercritical CO2 microemulsion. By mixing two microemulsions containing Ag+ and X- ions separately, silver halide (AgX) nanoparticles were also synthesized in supercritical CO2. Formation of these nanoparticles was monitored spectroscopically using a high-pressure fiber-optic reactor equipped with a CCD array W-Vis spectrometer. The water-in-CO2 microemulsion mar provide a reaction system for a wide range of nanomaterials syntheses involving ionic species that are normally insoluble in supercritical CO2. This system may have profound implications for nanoparticle synthesis and catalysis in supercritical CO2 with virtually no detrimental environmental effects.