The phase behavior of 1-glyceryl monooleyl ether (GME) in mixtures of propylene glycol (PG) and water was investigated by visual inspection, polarization microscopy, small-angle X-ray diffraction, and conductance measurements. A phase diagram, based on over 200 samples of the ternary system GME-PG-water, was constructed at 20 degrees C. Without PG, GME forms a reverse micellar phase with up to 10 wt % water and a reverse hexagonal liquid-crystalline phase between 10 and 25 wt % water, a phase that can coexist with excess water. If PG is added in amounts exceeding about 10 wt %, then cubic and lamellar liquid-crystalline phases start to form. A cubic phase, belonging to space group Pn3m, can coexist with excess PG-water mixtures. If even more PG is added, then the cubic phase is transformed into a sponge phase. A lamellar phase forms at water contents between 10 and 15 wt % and with widely differing PG/GME weight ratios. We postulate that the phase behavior is caused by the fact that PG makes the interfacial region between self-assembled GME and PG-water less negatively curved, which in turn allows for the formation of the new phases. The phase behavior obtained for the GME system shows a striking similarity with the phase behavior of the corresponding system in which the GME has been replaced by the ester, 1-glycerol monooleate (GMO), differing only in one extra carbonyl oxygen. The major difference is the lower amount of water present in the GME phases, an effect that is mainly due to the more hydrophobic character of GME compared to that of GMO.