The effect of solvent selectivity on the micelle structure formed by a poly(styrene-b-ethylene-alt-propylene) diblock copolymer (PS-PEP, M-PS = 42,000 g/mol and M-PEP = 62,000 g/mol) in solutions of squalane, a highly selective solvent for the PEP block, and 1-phenyldodecane, a neutral solvent, is investigated using dynamic light scattering (DLS) and small-angle Xray scattering (SAXS). The results reveal that addition of 1-phenyldodecane systematically reduces solvent selectivity, leading to decreases in the critical micelle temperature (T-CMT) and the average number of chains in a micelle (N-agg), and an increase in the solvent fraction in the core. However, the core blocks have essentially unperturbed dimensions (i.e., R-c approximate to 2 x < R-g >(o), where < R-g >(o) is unperturbed radius of gyration of the core block), and therefore the micelle core radius (R-c) is nearly independent of solvent selectivity, which has not been anticipated by previous theoretical descriptions. These results demonstrate that under such conditions, excluded volume effects in the corona blocks play the dominant role in determining micelle structure. (C) 2015 Wiley Periodicals, Inc.