Laser light scattering (LLS) techniques were used to characterize the micellization of poly(D,L-lactide)-poly(ethylene glycol)-poly(D,L-lactide) (LEL) triblock copolymer (MW 1K-2K-1K) in aqueous solution. We observed the existence of both thermodynamically stable flower-like micelles (regular micelles) and large, less soluble nanoparticles (irregular micelles) in dilute aqueous solutions with the same preparation procedure. Both kinds of micelles were found to co-exist with single copolymer chains. The initial copolymer concentration determines the nature of the micelles. The regular core-shell micelle formation follows a closed association mechanism, resulting in flower-like micelles. The hydrophobicity of a L unit is estimated as similar to0.5-0.6 B (polyoxybutylene) units from the micellization parameters, which is quite consistent with earlier estimations obtained from EL diblock copolymers. (C) 2004 Elsevier Ltd. All rights reserved.