We demonstrate that nanometre-sized polymer vesicles (polymersomes) assembled from two dissimilar diblock copolymers can undergo shape changes, driven by strong lateral polymer/polymer segregation within the membrane. The two particular block copolymers consisted of identical hydrophobic fragments to stimulate co-assembly, while their hydrophilic segments were either neutrally or negatively charged. It is hypothesized that demixing of the two types of polymer amphiphiles within the bilayer is caused by the different hydrophilic polymer fractions exhibiting intrinsically different interfacial curvatures upon self-assembly. Evidence for spontaneous domain formation came from dynamic light scattering analysis, electron microscopy and a fluorophore/quencher system. Given the vast amount of possible polymeric amphiphiles that can be co-assembled into hybrid aggregates, many more interesting and possibly biomedically relevant materials can be generated once inter-membrane phase separation becomes well-characterised and understood.