The block length dependence of the morphological phase diagram was investigated for PS-b-PAA copolymers in dioxane/water using transmission electron microscopy and light scattering techniques. The study was performed on copolymers with PS block lengths ranging from 49 to 310 repeat units and PAA block lengths from 7.2 to 26 repeat units. The water content ranged from 0 to 50 wt % and the copolymer concentration from 0 to; 10 wt %. For all the copolymers with increasing water content, the sequence of morphologies, in general, follows the order of single chains, spheres, sphere and rod mixtures, rods, rod and bilayer mixtures, pure bilayers, and finally mixtures of bilayers and inverted structures. The bilayers here include vesicles, lamellae, and more complicated structures. It was found that the boundaries of various morphological transitions generally shift to lower water contents with increasing total block length and with decreasing PAA block length. Most importantly, it was found that long core-forming blocks and high water contents favor the formation of vesicles and that short core-forming blocks and low water contents favor open bilayers (e.g., lamellae). This finding suggests that in block copolymer systems the increase of bending modulus favors vesicle formation, which reinforces the conclusion of a theoretical study for mixed small molecule surfactant systems. The Gibbs free energies for the single chain to sphere transition were also estimated from the morphological phase diagram. It was found that at the same water content the free energy becomes less negative both with decreasing total block length and with increasing PAA block length.