PI cores of the micelles of poly(styrene-b-isoprene) (PS-b-PI) diblock copolymers, in PS selective solvents, were cross-linked with sulfur monochloride (S2Cl2). The cross-linked micellar structure was maintained after dialysis in THF (neutral solvent) and did not change during heating. Cross-linking brought about the opportunity for TEM images in a solution state; otherwise, the micellar structure would be destroyed (or changed) during the evaporation of the solvent on a carbon-coated copper grid. The Flory interaction parameter, chi, between the PI block and the solvent was controlled by mixing two selective solvents (DMP/toluene, DMP/DEP and DEP/DBP) which have different degrees of selectivity for the PS block, as well as heating the solutions. Two block copolymers, PS(7.2K)-b-PI(7.8K) and PS(5.5K)-b-PI(18.8K), were studied in order to clarify the effects of the relative chain length of each block on the micelle structure in the selective solvents. PS(7.2K)-b-PI(7.8K), which is nearly symmetric, showed only spherical micelles in the DMP/toluene mixture. The basic spherical micellar shape of PS(7.2K)-b-PI(7.8K) did not change with chi, while the size and aggregation number of the micelles increased as chi increased until 2.05 and then were saturated after that. PS(5.5K)-b-PI(18.8K), which is asymmetric, showed a structural change from spherical to cylindrical to vesicular micelles with an increase in the selectivity of the DMP/DEP and DEP/DBP mixtures (which was also confirmed by TEM and SAXS studies). Giant vesicular micelles with a diameter of similar to 2.5 mu m were observed in high-selectivity solvents. The size of the vesicular micelle seemed to decrease as selectivity decreased. The systematic changes of the micellar structures of PS(5.5K)-b-PI(18.8K), via changes in solvent selectivity, could be demonstrated through TEM images, which were prepared by evaporating the solvent of the cross-linked micellar solution onto the carbon-coated grid after dialysis.