We report about aqueous self-assembly of binary mixtures of poly(ethylene oxide)-block-polycaprolactone-block-poly(2-methyl-2-oxazoline) (PEO-b-PCL-b-PMOXA) triblock copolymers containing identical PEO (45 units) and different PCL (48-153 units) and PMOXA (325 units) block lengths. All mixtures targeted a certain triblock copolymer; that is, mixtures had the same average block lengths as the targeted polymer. Self-assembled morphologies formed by mixtures were compared with the morphologies formed by the corresponding targeted polymers, which were polymersomes or cloudlike aggregates. The mixtures were prepared using premixing and postmixing protocols. In the premixing protocol, polymers were mixed before self-assembly. In the postmixing protocol, structures were mixed after self-assembly. Self-assembly was performed at 62 degrees C using the film rehydration method, and the microscale structures were visualized using laser scanning microscopy. In postmixing protocol, polymersomes stayed intact, whereas cloudlike aggregates/partially dissolved polymer/irregularly shaped particles mixed with each other. When polymersomes were mixed with cloudlike aggregates coexisting with polymersomes, we observed formation of multicompartment polymersomes through uptake of one kind of polymersomes by another. In premixing protocol, most of the mixtures resulted in a structure formed by a single targeted polymer. A few exceptions were self-assembly of polymersomes instead of cloudlike aggregates, and vice versa, and formation of multilamellar polymersomes and onionlike structures.