Ionic species can be stabilized in the water core of a water-in-CO₂ microemulsion in supercritical fluid CO₂; by mixing two microemulsions containing Ag⁺ and X⁻ ions separately, silver halide (AgX) nanoparticles were synthesized in supercritical CO₂; the water-in-CO₂ microemulsion may provide a reaction system for a wide range of nanoparticle syntheses involving ionic species in supercritical CO₂.