Micellar structures of amphiphilic poly(1,2-butadiene)-block-poly(ethylene oxide) diblock copolymers have been crosslinked in aqueous solution by irradiation. By transmission electron microscopy (TEM) of negatively stained specimens it is shown that the predominant structures present are copolymer vesicles (which appear to be double-layered). These fixed vesicles are able with respect to their shape and can be transferred from water into a good solvent for both blocks, such as ferrahydrofuran, thus demonstrating the effectiveness of the crosslinking. In addition to the vesicles, a small number of flexible cyclindrical/filamentous structures, sequentially fused vesicles/strings of vesicles and giant sheet-like vesicles are also visible after cross-linking. The vesicle chains seem to be formed by fusion of the double-layered vesicles; the outer layer of the vesicles apparently fuses sequentially, whereas the inner shell remains intact, creating periodic linear thickenings.