Temperature-induced vesicle-to-micelle transitions of polystyrene-block-poly acrylic acid (PS139-b-PAA17) aggregates in tetrahydrofuran (THF)/H2O solvent mixtures are studied. For a typical system with an initial concentration of PS139-b-PAA17 of 2 wt% and 50 vol% of H2O, the morphology of the aggregates changes from vesicles to micelles upon heating from room temperature to 45 °C. The transition temperature is found to depend on the polymer concentration as well as solvent composition. A higher polymer concentration results in a higher transition temperature. The morphological change is attributed to a change in the solvent-polymer interactions, which results in a reduction in interfacial energy. The corresponding temperature-induced morphological change is employed as a strategy for the reversible release and encapsulation of small molecules. The release of Rhodamine 110 bisamide above the transition temperature is observed as a result of the trypsin-catalyzed hydrolysis of the bisamide into Rhodamine 110. Likewise, the successful encapsulation of Rhodamine 110 below the transition temperature is proven using sodium nitrite as a chemical quencher.