The phase diagram of the phospholipid, POPC, the non-ionic surfactant, C 12 E 2 , and deuterium oxide was determined at room temperature using 31 P and 2 H NMR as well as Polarising Optical Microscopy. Lamellar phase is present over the largest area of the triangular phase diagram. Inverted hexagonal, H II , and inverted micellar, L 2 , phases are present at low water contents although neither the lipid nor the surfactant show H II phase in binary aqueous systems. There are biphases separating these single ones. V 2 cubic phases have been characterised in a lamellar plus cubic biphase in the surfactant rich part of the phase diagram. Upon heating it forms single cubic phases. A qualitative description of the phase transitions and the corresponding intermolecular interactions is presented.