Semiconductor nanopaparticles of CdS and ZnS were synthesized by mixing two water-in-CO2 microemulsions with one containing S2- ions and the other containing Cd2+ or Zn2+ ions in the water core. Nanoparticle formation was monitored in situ by measuring their absorption spectra in the UV−vis range using a high-pressure fiber-optic system. The size of the nanoparticles formed in the microemulsion was found to depend on the water-to-surfactant ratio (W). At W = 6 and 12, the band gaps of CdS were calculated to be 3.50 and 3.15 eV, corresponding to particle radii of 1.4 and 1.7 nm, respectively. The ZnS particles synthesized at W = 12 showed a band gap of 4.26 eV, corresponding to a mean particle radius of 1.6 nm. This microemulsion-plus-microemulsion approach offers a simple method for synthesizing various nanoparticles in supercritical CO2 using water-soluble reagents as starting materials.