The effect of a polymer oil, polydimethyl siloxane, on the phase behavior of the water/C(12)EO(6)/isopropyl myristate (IPM) system has been studied. Since the polymer oil is completely soluble in IPM, it was dissolved in IPM and the solutions were used as the oil phase to study its effect. The presence of polymer increases the hydrophobic nature of IPM and thereby decreases the solubilization of oil into the surfactant phase (microemulsion). Moreover, at a certain range of silicone oil concentration in the IPM, a four-phase body consisting of excess water, excess oil, and two surfactant phases is formed within the ternary system. The two surfactant phases are designated as D (with bicontinuous type of structure) and D’ (with L(3) type of structure). Careful phase behavioral studies revealed that, with increasing silicone oil concentration, a three-phase region containing water phase, D’ phase, and an oil-rich D phase develops within the system and this region overlaps with the normal three-phase region of the system containing the water, D, and oil phases to give a four-phase body. The order of various phases from top to bottom in the four-phase body is oil, D, D’, and water.