The phase diagrams for tetramethyldioctyldisiloxane (TDDS), water and several organic components were determined using refractive index, polarizing microscopy and low angle x-ray diffraction. The solubility limits of the alcohols were established by quantitative analysis and compared to the refractive index of known single phase mixtures. The non-ionic surfactants systems contained isotropic liquids of varying concentrations. Two lyotropic liquid crystal regions were discovered, one of lamellar structure and one consisting of a hexagonal array of closed packed cylinders. The character of the amphiphiles components to solubilize the TDDS is reflected in Ihe phase equilibria.