H-1 NMR, 2D NOESY, and spin-lattice relaxation time (T-1) measurements were used to investigate the aqueous solution behavior of poly(ethylene glycol) monomethyl ether 3,5-dinitrobenzoate (PEG-DNB). A cloud point diagram revealed a phase behavior consisting of two phase separation processes, one, at high temperature, common for solutes containing oxyethylene groups whose solubility in water decreases with temperature, and the other, at low temperature, driven by the hydrophobicity of the DNB group. The intramolecular charge-transfer complexation between the ether oxygens from the PEG chains, acting as p-electron donors, and the dinitrobenzoyl group, which is a pi-electron acceptor, was found to be the cause of the unusual phase behavior and H-1 NMR multiplet patterns. The molecular weight selectivity of the chemical shift was correlated to the different conformations adopted by complexes having different PEG lengths. These conformations are the result of the competition between hydration forces on one hand, and CT interactions and hydrophobicity of DNB on the other. In addition to complexation, there are large aggregates forming due to the amphiphilic character of these molecules. To explain the coexistence of aggregation and coiled complex conformation a model is proposed, consisting of supramicellar composite (multicenter) aggregates formed by the association of many PEG-DNB coils, each being unimolecular micelles. The composite aggregates are characterized by a hydrophobicity gradient associated with the molecular weight distribution along the aggregate radii.