Both laser light scattering (LLS) and small-angle X-ray scattering (SAXS) were used to study the water-induced formation and structure of micelles and supramolecules of Pluronic P103 [(EO) 17 (PO) 60 (EO) 17 ] in o-xylene, a selective solvent for the long middle block. In pure o-xylene, P103 molecules exist as unimer coils with an equivalent hard-sphere radius of 1.6 nm even at fairly high concentrations. Micelles with a PEO/water core and a PPO dominated corona were formed in the presence of water when the P103 concentration ¢0.046 g/mL. The size and structure of micelles have been studied as a function of solubilized water content Z (the molar ratio of water to EO units) in micelles. The micelles change from a somewhat open structure with some EO units either dangling out of the micellar core or being incorporated into neighboring micellar cores at low Z values to a flower-like structure with relatively sharp interface at high Z values. At low Z values ( õ about 2.9), micelles tend to have a structure with part of the poorly solvated PEO blocks present in the corona. With more water added to the core, the PEO blocks in the corona gradually entered into the core, and the PPO blocks backfolded to form loops. With increasing Z, the micellar core radius, R c , and the hard-sphere volume fraction, f, of micelles increased; the aggregation number, N, kept nearly a constant; but the hydrodynamic radius, »R h … 0 , and the corona thickness, R s , decreased. At high Z values ( ú about 2.9), micelles have a flowerlike structure with the two end PEO blocks belonging to the small micellar core. With increasing Z, the values of R c , f, and N increased, while R s kept nearly a constant. In the concentrated regime ( C ú 0.30 g/mL), a stiff polymer network at a critical f value of 0.49 was formed. The supramolecular structures with a face-center cubic packing, and a possible hexagonal packing at higher polymer concentrations (i.e. ú 0.55 g/mL), were observed, respectively.