Three diblock copolymers, each consisting of a relatively polar polyoxometalate-containing block and a nonpolar polythiophene block, were studied for their solution self-assembly behavior. Hollow spherical vesicular structures have been observed; their formation is driven by solvophobic–solvophilic interactions. The size of the vesicles can be controlled by varying the polarity of the solvent. Regular vesicles seem to form when the solvent combination is relatively polar, while reverse vesicles form when the solvent is relatively nonpolar.