Thermodynamically stable microemulsions are formed in a water(brine)/ionic surfactant/cosurfactant/polymer-oil(silicon oil having an NH 2 group) system. The effect of added salt on the formation of microemulsions is remarkable. The cosurfactant/ionic surfactant ratio to give the maximum solubilization decreases with the increase in salinity. The minimum concentration of surfactant (ionic surfactant + cosurfactant) to make equal amounts of water and polymer oil a single phase is decreased upon addition of salt. The salt also has the effect of destroying lamellar liquid crystal which turns into isotropic microemulsions. The electrical conductivity data show that water-continuous, oil-continuous, and bicontinuous microemulsions are formed in this polymer-oil system. Microemulsions were not found in a polydimethylsiloxane system at a water/oil ratio of unity. Hence, the NH 2 group in silicone oil is important to form microemulsions. The hydrophilic groups of silicone oils may penetrate to the palisade layers of ionic surfactant and cosurfactant to stabilize microemulsions.