We have used the highly purified, homogeneous trimethylsilane surfactant CH3Si(CH2)(6)(OCH2CH2)(5)OCH3 to determine the full binary phase diagram with water. The trimethylsilane surfactants display similar properties to those of the well-known superspreading trisiloxane surfactants but have higher hydrolytic stability. Although the richness and sequence of phases found for the present system resemble that known from water-nonionic surfactant systems of the alkylpolyglycolether type, a peculiarity is observed: there are two disconnected lamellar phases, a concentrated one and a dilute one. Furthermore, an L-3 phase is found located at lower concentrations than the dilute lamellar phase. The position, the extent, and the phase sequence around the L-3 and dilute lamellar phases allows a clear assignment of seven two-phase regions, which require the existence of four triple lines. Each two-phase region has been identified, and all, triple lines have been located to within a few hundredths of a degree.