The phase behavior and microstructure of C12E6/ether oil/water mixtures have been examined systematically as a function of increasing oil polarity. As the ratio of ether linkages to methylene groups of the ether oils is increased, the oils become more water soluble and more polar (less hydrophobic). When ethylene glycol dibutyl ether is replaced with the more polar ethylene glycol diethyl ether in C12E6/ether oil/water mixtures, the three-phase region shrinks and disappears: the system bypasses a tricritical point. Simultaneously, the liquid crystalline region retreats to higher surfactant concentration, and light and X-ray scattering measurements indicate that the microstructure decreases dramatically in size. According to small-angle X-ray scattering results, C12E6/ethylene glycol diethyl ether/water mixtures retain an interface between oillike and waterlike domains. However, since the ether is highly soluble in the water and interface domains, the microstructure is diffuse.