Two amphiphilic diblock copolymers were mixed and found to form giant vesicles with laterally segregated domains. A-B and C-B diblocks with a similar hydrophobic core block B were blended in various proportions and generated polymersomes with micron-scale domains that could be seen via fluorescent A*-B diblock using confocal microscopy. With increasing proportion of A-B, the fluorescent domains grow in area fraction. Segregation appears to be driven by the fact that C-B can be crosslinked by added ligand, which promotes demixing of C-B and A-B to form laterally segregated domains. Studies of pure A-B and C-B vesicles substantiate these descriptions and, in the case of C-B vesicles, also demonstrate a solid-fluid transition based on ligand concentration and pH.

**Christian and Tian are equal contributors to this work.