We reported aqueous solutions of poly(caprolactone-b-ethylene glycol-b-caprolactone) (PCL-PEG-PCL) that underwent sol-gel-sol transition as the temperature increased (Macromolecules 2005, 38, 5260-5265). However, when the triblock copolymer aqueous solution (20 wt %), initially as a sol phase, was left at room temperature (20 °C), it turned into an opaque gel in 1 h. The crystallization of the PCL-PEG-PCL triblock copolymer in water was suggested to be responsible for such a kinetic aspect of the phase transition. In addition, PEG/PCL multiblock copolymers were synthesized by coupling the triblock copolymers using terephthaloyl chloride. Even though both PCL-PEG-PCL triblock and PEG/PCL multiblock copolymer aqueous solutions (20 wt %) instantaneously undergo a sol-to-gel transition upon injection into 37 °C water and their thermogels show a maximum modulus at around body temperature (35-42 °C), the multiblock copolymer shows a pronounced sol phase stability at room temperature. The fundamental difference in such phase behavior between triblock and multiblock copolymers seems to lie in their ability to form micelles at low temperature and high crystallizability of the low molecular weight PCL.