The isothermal phase diagram at 25 degrees C of ternary mixtures of 1-palmitoyl-2-oleoyl-phosphatidylcholine tetraoxyethylene dodecyl ether, and (H2O)-H-2 has been determined by P-31 and H-2 NMR. The liquid crystalline lamellar phase (L(alpha)) occupies the largest area in the triangle. It extends from the lipid-water to the surfactant-water axis. Water separation (L(alpha) + W) occurs at high water concentrations. A surfactant-rich isotropic phase (L(2)), an inverted hexagonal (H-II), and a gel phase (L(beta)) are observed at low water content depending on the lipid and surfactant concentrations. Most of the phase transitions occur via two-phase regions. A small three-phase region is detected in the water-poor and surfactant-rich region. A second three-phase region should exist at low water and surfactant concentrations according to the phase rule. Simple model considerations allow for a qualitative understanding of some of the phase transitions observed. The L(alpha)/L(alpha) + W boundary at low surfactant content is calculated by assuming an ideal mixing behavior. The mesophases formed in the water-poor region are discussed with respect to the molecular shape of the amphiphiles. Deuterium quadrupolar splitting measurements using partially deuterated surfactant are consistent with a relocation of the surfactant relative to the lipid/water interface at the lamellar to hexagonal phase transition.