Ionic species can be stabilized in the water core of a water-in-CO(2) microemulsion in supercritical fluid CO(2); by mixing two microemulsions containing Ag(+) and X(-) ions separately, silver halide (AgX) nanoparticles were synthesized in supercritical CO(2); the water-in-CO(2) microemulsion may provide a reaction system for a wide range of nanoparticle syntheses involving ionic species in supercritical CO(2).