The partial ternary phase diagram was investigated for the polystyrene(310)-block-poly(acrylic acid)(52) copolymer in dioxane/water mixtures in regions in which self-assembled nanoaggregates of various morphologies are seen. Both fractionated and unfractionated copolymers were used; the unfractionated copolymer contains homopolystyrene. The study was carried out over the range of water contents from 0 to 45 wt % and copolymer concentrations from 0.1 to 10 wt %. Freeze-drying transmission electron microscopy (TEM), turbidity measurements, as well as static and dynamic light scattering were employed. Because of the proximity of the melting points and boiling points of water and dioxane, quenching and subsequent freeze-drying of solution samples can be employed to preserve aggregate morphologies, The morphologies can then be observed using TEM. The reversibility of various morphological transitions was examined by means of TEM and turbidity measurements. With increasing water content, the sequence of copolymer structures in solution follows the order of single chains, spheres, sphere and rod mixtures, rods, rod and vesicle mixtures, and finally pure vesicles. The morphologies observed here are under thermodynamic control. Not only the water content but also the polymer concentration affects the morphologies and the sizes of the aggregates. For the unfractionated polymer, the single-chain/sphere boundary shifts to lower water contents relative to that of the fractionated copolymer, while the other morphological boundaries move to higher water contents. On the basis of the progressive changes of the aggregate morphologies with the addition of water, possible pathways of the morphological transitions are suggested and discussed briefly. Also, approximate thermodynamic functions are estimated for the morphological transitions on the basis of the morphological boundaries. The combination of freeze-drying TEM techniques with turbidity measurements is very useful in exploring the morphological behavior of block copolymers in solution.