Solution property of hydrogenated polystyrene-b-poly(ethylene/butylene)-b- polystyrene triblock copolymer (SEBS copolymer) was studied by using static light scattering and dynamic light scattering for cyclohexane and N-methylpyr-rolidone (NMP) solutions. From the values of dimensionless parameters rho, defined as the ratio of radius of gyration < S-2 >(1/2) to hydrodynamic radius R-H, and solubility parameters, SEBS copolymer proved to exist as single chain close to random coil in nonpolar cyclohexane, whereas aggregate into the core-shell micelle consisting of poly(ethylene/butylene) (PEB) core surrounded by PS shell in polar NMP. The core-shell micelle formed in NMP is composed of 65 polymer chains, having three times larger average chain density (d = 0.12 g cm(-3)) than a single polymer chain (d = 0.04 g cm(-3)) in cyclohexane. The comparison with the aggregation behaviors in other solvents demonstrated that the aggregate compactness of the copolymer depended largely on solvent polarity, resulting in formation of the highly dense PEB core (R-c = 4.5 nm) and the thick PS shell (Delta R = 22.9 nm) in high-polar NMP. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 588-594, 2010