We have investigated the self-assembly behavior of an amphiphilic diblock copolymer, polystyreneblockpoly(ethylene oxide) (PS-b-PEO), in N, N-dimethylformamide (DMF)/water and DMF/acetonitrile. In both cases water and acetonitrile are selective solvents for the PEO block. The degrees of polymerization of the PS and PEO blocks were 962 and 227 (PS962-b-PEO227), respectively. Micelle morphologies of the block copolymer in both systems could be controlled by varying copolymer and selective solvent concentrations. With increasing the water concentration in the DMF/water or the acetonitrile concentration in the DMF/acetonitrile system, the micelle morphology observed in transmission electron microscopy changed from spheres to wormlike cylinders and then to vesicles. The morphological diagrams were constructed from the study of the micelle morphology changes in different copolymer concentrations and the critical micellization concentrations for both systems at different copolymer concentrations as determined by static light scattering experiments. In between the concentration regions of two neighboring pure micelle morphologies, mixed morphologies such as spheres with short cylinders or wormlike cylinders with vesicles could be found. Although the trend in morphological changes was identical in these two systems, there were remarkable differences in the morphological diagrams of PS962-b-PEO227 with respect to the percentage of selective solvent added. This is due to the large difference between the polymer-selective solvent interaction parameters. On the basis of the observations of morphological reversibility and annealing experiments, these two morphological diagrams were proven to be in thermodynamic equilibrium. The driving force for these morphological changes was understood to approach micelle free energy minimization. Approximate micelle free energy calculations confirmed that the free energy decreases as the morphology changes from spheres to wormlike cylinders and then to vesicles with an increase in the selective solvent concentrations. Possible change mechanisms are also discussed.