The formation of CuS nanoparticles from Cu(NO3)(2) and Na2S by the mixing of two separate water-in-carbon dioxide (W/C) microemulsions supported by the sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate (di-HCF4) was investigated. At a water-to-surfactant ratio of 10 and a copper concentration of 7.8 mM in the aqueous phase, the size of copper sulfide particles ranged from 4 to 6 nm. The temperature-pressure phase diagrams of W/C microemulsions at various electrolyte concentrations in. the aqueous phase were also determined. The presence of the electrolytes was found not to affect the cloud-point pressures significantly, even at high concentrations.