Vesicles prepared from block copolymers have been mooted for the encapsulation of water-soluble molecules. This is because the membranes of polymer vesicles have been shown to be more stable than those in vesicles formed from lipids, with the membrane properties being tuned by the length and nature of the hydrophobic block in the polymer. The generally accepted mechanisms of vesicle formation involve either wrap-up of a lamellar sheet or formation via a sequence of micelle to worm to disks to vesicles. These should lead to efficient encapsulation. Alternatively, a method involving phase separation followed by re-structuring has been recently suggested. Here, we show that this final mechanism holds for vesicles formed from a PEO-b-PDEAMA copolymer by a pH switch and that this mechanism leads to highly inefficient encapsulation on vesicle formation.