Poly(ethylene glycol)-b-polycaprolactone (MPEG-PCL) diblock copolymers were synthesized via a ring-opening polymerization of epsilon-CL monomers with MPEG as an initiator. Their solubilities and apparent critical micelle concentrations (CMC) in aqueous solution were investigated as well as the determination of the micellar hydrodynamic diameter using dynamic light scattering (DLS). As PCL block length increased, the solubility and CMC decreased while diameters of micelles increased. The gel-sol transition behaviors were investigated using a vial tilting method. Aqueous solutions of copolymers undergo a gel to sol transition with increase in temperature when their polymer concentrations are above a critical gel concentration (CGC). The CGC of the copolymers and gel-sol transition temperature are influenced by the PCL chain length. The tapping mode AFM was performed by imaging the freeze-dried deposits from the copolymer solutions on mica to investigate a process from free chains to micelles and to gel. (c) 2006 Wiley Periodicals, Inc.