Diblock copolymers composed of a water-soluble polyphosphobetaine block with a short chain length and either a cationic or anionic block were prepared. Giant polyion complex vesicles (G-PICsomes), with a size on the order of micrometers, were prepared by dialysis from a mixture of oppositely charged diblock copolymers in a 1.5M NaCl aqueous solution against pure water. Hydrophilic guest molecules could be encapsulated into the G-PICsomes.