The micelles of polystyrene-block-poly(acrylic acid) (PS154-b-PAA49) are made to transform slowly in mixed solvent, allowing continual trapping of the various intermediates. During the transformation from cylindrical to vesicular micelles, it appears that a section of the cylinder first flattens to give a lamellar section, which then depresses to give a bowl-like moiety, before finally converting to a fully enclosed vesicle. Most part of this transformation involves the “flow” of polymer domains without mingling of the hydrophobic and hydrophilic domains. On the bases of the literature and the observation in this work, it is proposed that the reduction of surface-to-volume ratio provides the thermodynamic driving force for the cylinder-to-vesicle transformation, whereas molecular reorganization within the polymer domains creates the kinetic barrier. From the point of view of molecular interactions, the “flow” of polymer domain involves a low barrier, whereas the merging of two micelles, the severing of cylindrical micelles, and the closing of partial vesicles encounter high barriers. Moreover, the kinetic barrier is reduced when PSPAA containing shorter PAA blocks is used, or when the PAA block are protonated. This mechanistic proposal explains the kinetically controlled transformation pathway and structural features of the observed intermediates.