Phase behavior was investigated for water/supercritical CO2 (W/scCO(2)) microemulsions stabilized with sodium bis(1H,1H,2H,2H-heptadecafluorodecyl)-2-sulfosuccinate (8FS(EO)(2)) mixed with various guest surfactants. Only for the mixtures with fluorocarbon-hydrocarbon hybrid anionic surfactants (FC6-HCn), the maximum water-to-surfactant molar ratio (W-0(C)) was larger than that estimated from linear interpolation of the W-0(C) values for pure 8FS(EO)(2) and pure guest surfactant. Fourier transform infrared (FT-IR) measurement for the microemulsion revealed that the mixing of 8FS(EO)(2) with FC6-HCn can prevent a phase transition from the microemulsion to the liquid crystal even in the presence of excess water. It was also found from the measurement of water/scCO(2) interfacial tension that the area occupied per surfactant molecule was markedly increased by the mixing with FC6-HCn. The loose molecular packing, probably due to a micro segregation of 8FS(EO)(2) and FC6-HCn, is consistent with the enhanced stability of the microemulsion upon surfactant mixing.