- The phase behavior of the mixed didodecyldimethylammonium bromide (DDAB)–poly(ethylene oxide) (PEO)–water system is investigated by optical microscopy and NMR methods at 298 K. DDAB forms two different lamellar liquid crystalline phases in water, and, at high water contents, prior to the formation of the first lamellar phase, a vesicular region is also detected. The solubilization of the nonadsorbing, water soluble polymer PEO in the phases formed by DDAB is limited for the lamellar phase regions, and up to 20 wt% of polymer can be solubilized in the binary vesicular phase. A segregative type of phase separation is observed between surfactant and polymer. The triangular phase diagram is dominated by multiphase heterogeneous regions. Rich microstructural transformations are found to occur upon addition of PEO into the vesicle region. Microstructural transformations are followed by a combination of methods including direct imaging of microstructure by cryogenic transmission electron microscopy (cryo-TEM) and water self-diffusion measurements by the pulse gradient field NMR method. At a concentration of 2 wt% of DDAB cryo-TEM detects multiwall, polydisperse vesicles in equilibrium with single-wall vesicles. Upon addition of PEO to the solution of 2 wt% DDAB the vesicles show a tendency to fuse at and above 8 wt% of PEO. In correspondence, an increase in viscosity and a decrease in water self-diffusion coefficient are registered. With further addition of polymer more vesicles aggregate to form a cluster-like structure. This behavior can be described in the framework of the theory of colloid–polymer interactions such as an attractive osmotic force between vesicles due to polymer depletion near the vesicles.