The effects of adding didodecyldimethylammonium bromide (DDAB) to mixtures of n-alkyl polyglycol ether (CiEj), water, and silicone oil are Systematically studied. In mixtures of C8E3 and C12E6 with water, small amounts of DDAB cause the upper miscibility gap to vanish and be replaced by a high-temperature lamellar phase. In mixtures of C12E5 with octamethylcyclotetrasiloxane (D4), the addition of DDAB makes the surfactant mixture more hydrophilic and expands the lower miscibility gap by increasing the (pseudo)critical temperature. In ternary mixtures of CiEj, D4, and water, adding DDAB increases the surfactant efficiency by up to a factor of three, expanding the single-phase microemulsion region to higher temperatures and lower surfactant concentrations. The temperature limits of the single-phase microemulsion correlate with changes in the pseudo-binary phase diagrams upon addition of DDAB. Additionally, the surfactant mixture stabilizes liquid crystalline regions, but at temperatures that do not obscure the gains in surfactant efficiency offered by the ionic-nonionic surfactant mixture. Similar results are reported for two linear silicone oils, hexamethyldisiloxane and decamethyltetrasiloxane.