The ternary phase behavior of the trisiloxane E-12 polyoxyethylene surfactant with water and three low molecular weight silicone oils has been determined. The silicone oils were the tetra- and pentacyclosiloxanes, D-4 and D-5, and the short linear tetrasiloxane, MD2M. Microstructures were investigated using small-angle X-ray scattering and polarized light microscopy. All three ternary systems exhibit similar phase behavior, forming surfactant-rich and water-rich microemulsions, and liquid crystal phases. They follow the now familiar (2) under bar Phi (Winsor I) to three-phase (Winsor III) sequence with increasing temperature. Because of the large immiscibility gap between this very polar trisiloxane surfactant and the silicone oils, (2) over bar Phi (Winsor II) type behavior was not observed, even at elevated temperatures (up to 140 degrees C). The cubic I-1 and I-3 phases were found in the ternary systems along with hexagonal H-1 and lamellar L-alpha liquid crystal phases, which persisted to substantially higher temperatures than their counterparts in the binary surfactant in water system.