The phase behavior of the poly(dl-lactide-co-glycolide) (PLG)/monoolein (MO)/1-methyl-2-pyrrolidinone (NMP)/water system has been studied. The particular system was selected because it was a candidate for being a suitable system for creation of lipid−polymer hybrid particles. Phase diagrams of the four ternary subsystems, as well as phase behaviors of selected quaternary mixtures, have been determined. The PLG/NMP/water system exhibits a behavior similar to a polymer/solvent/nonsolvent system, and the PLG/MO/NMP system shows a segregative behavior similar to a polymer 1/polymer 2 (or surfactant)/solvent system. The ternary MO/NMP/water subsystem exhibits a rich, mainly mesomorphic phase behavior, including lamellar and cubic phases, and offers thus possibilities to create both liposomes and cubosomes. At high NMP content, the system gives rise to several liquid phases, such as a sponge phase and MO-poor and MO-rich liquid phases. Addition of PLG to the MO/NMP/water subsystem reveals that both liquid crystals and liquid phases may exist in equilibrium with PLG-rich liquids, the latter being highly viscous. At high water content, the PLG phase may be caught in a glassy state at room temperature. The Flory−Huggins theory has been used to model three of the ternary subsystems and some parts of the quaternary system.