The association and surface properties of three oxyethylene/oxyphenylethylene (ES) diblock copolymers in aqueous solution have been studied. Copolymers E50S3.5, E50S5.1, and E51S6.5 were synthesized and characterized by gel permeation chromatography and 13C NMR spectroscopy. Surface tensiometry was used to determine critical micelle concentrations (cmc) at several temperatures, and thereby values of the enthalpy of micellization (ΔmicHo). The values of cmc and ΔmicHo were found to be low (e.g., 0.03 g dm-3 at 30 °C and 12 kJ mol-1 for E51S6.5), which is consistent with the high hydrophobicity of oxyphenylethylene block. Dynamic and static light scattering were used to characterize the micelles in dilute solution, yielding micellar association number, hydrodynamic and thermodynamic radius, and the related thermodynamic expansion factor. At higher concentrations, solution of copolymers E50S5.1 and E51S6.5 formed hard isotropic gels.