A poly(ethylene glycol)-b-poly(L-lysine) diblock copolymer (PEG-b-PLL) was synthesized. Micellization of this hydrophilic copolymer due to the block-specific threading of alpha-cyclodextrin (alpha-CD) molecules onto the polyethylene glycol (PEG) block yielded supramolecular-structured nanoparticles, which undergoes pH-inducible gelation in aqueous media. The pH-inducible gelation of supramolecular micelle in water appeared to be completely reversible upon pH changes. The synergetic effect of selective complexation between PEG block and alpha-CD and the pH-inducible hydrophobic interaction between PLL blocks at pH 10 was believed to be the driving force for the formation of the supramolecular hydrogel. H-1 NMR and wide angle X-ray diffraction (WAXD) were employed to confirm the inclusion complexation between alpha-CD and PEG block. Meanwhile, the morphology of the micellized nanoparticles was investigated by transmission electron microscopy (TEM). The thermal stability of inclusion complexes (ICs) was investigated and the rheologic experiment was conducted to reveal the micelle-gel transition. Such pH-induced reversible micelle-gel transition of the supramolecular aggregates may find applications in several fields, for example as advanced biomedical material possessing stimulus-responsiveness. (C) 2008 Wiley Periodicals, Inc.