In this study, we report a new phenomenon dealing with the size-dependent behavior of the wall thickness of block copolymer vesicles, especially the decrease in wall thickness with decreasing vesicle size. Four vesicle-forming copolymers from the polystyrene-h-poly-(acrylic acid) (PS-h-PAA) family (i.e., PS(500)-b-PAA(50), PS(310)-h-PAA (28), PS(240)-b-PAA(15), and PS(412)-b-PAA(46)) were chosen for study. The sizes and wall thicknesses of the vesicles after quenching were determined front the TEM micrographs, and plots were made of the wall thickness versus size for each family of vesicles made from each of the various blocks. First, the effect of the length of the PAA block oil the relationship between the wall thickness and the size was examined. In the vesicles prepared from PS(500)-b-PAA(50), the copolymer with the longest PAA block that yields the smallest vesicles, the wall thickness decreases strongly with decreasing size. By contrast, in the case of vesicles made from PS(240)-b-PAA(15), for which a wide size distribution is obtained, only a weak size dependence of the wall thickness is seen. For vesicles made from the copolymer with intermediate PAA block length (i.e., PS(310)-b-PAA(28)), both strong and weak behavior regions are observed depending oil the vesicle size range. We suggest that this new phenomenon of the size dependence of the wall thickness can be considered to be another stabilization mechanism for very small vesicles, under conditions where chain segregation is insufficient to stabilize the size, The vesicles call be stabilized by decreasing the wall thickness for very small vesicles, resulting in the increase in area per corona chain, thus decreasing the corona repulsion on the inside. The effects of additives such as NaCl, HCl, or NaOH on the relationship between the wall thickness and the size were also investigated. By shielding the electrostatic repulsion among corona chains in the presence of NaCl, the strong behavior of the vesicles prepared from PS(412)-b-PAA(46) changes to a weak one as the width of the vesicle size distribution increases. In a NaCl concentration region around 10 mM, an opposite effect is seen relative to that observed in small vesicles in that the wall thickness decreases with increasing vesicle size for vesicles larger than ca. 300 rim, all effect ascribed to corona repulsion among the external corona chains, The addition of HCl also drives the relationship to be weaker through the protonation of the carboxylate groups of PAA chains, in all effect similar to that of NaCl. The presence of NaOH is expected to strengthen the relationship via the deprotonation of PAA, which increases the corona repulsion. However, because of the very short length of PAA chains in the system where a weak effect is seen, no significant effect of NaOH addition was observed because the size distribution remained broad.