The spontaneous hydrogel formation of a sort of biocompatible and biodegradable amphiphilic block copolymer in water was observed, and the underlying gelling mechanism was assumed. A series of ABA-type triblock copolymers [poly(D,L-lactic acid-co-glycolic acid)-b-poly(ethylene glycol)-b-poly(D,L-lactic acid-co-glycolic acid)] and different derivatives end-capped by small alkyl groups were synthesized, and the aqueous phase behaviors of these samples were studied. The virgin triblock copolymers and most of the derivatives exhibited a temperature-dependent reversible sol-gel transition in water. Both the poly(D,L-lactic acid-co-glycolic acid) length and end group were found to significantly tune the gel windows in the phase diagrams, but with different behaviors. The critical micelle concentrations were much lower than the associated critical gel concentrations, and an intact micellar structure remained after gelation. A combination of various measurement techniques confirmed that the sol-gel transition with an increase in the temperature was induced not simply via the self-assembly of amphiphilic polymer chains but also via the further hydrophobic aggregation of micelles resulting in a micelle network due to a large-scale self-assembly. The coarsening of the micelle network was further suggested to account for the transition from a transparent gel to an opaque gel. (c) 2006 Wiley Periodicals, Inc.