This work reports for the first time a simple and effective approach to trigger a spheres-to- vesicles morphological transition from amphiphilic block copolymer/polyelectrolyte complexes in aqueous solution. Vesicles and large compound vesicles (LCVs) were prepared via complexation of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) with poly(acrylic acid) (PAA) in water and directly visualized using cryo-TEM. The complexation and morphological transitions were driven by the hydrogen bonding between the complementary binding sites on the PAA and PEO blocks of the block copolymer. The findings in this work suggest that complexation between amphiphilic block copolymer and polyelectrolyte is a viable approach to vesicles and LCVs in aqueous media.