Structures of poly(ethylene propylene)-poly(ethylene oxide) (PEP-PEO) block copolymer micelles were determined from small-angle X-ray scattering and static light scattering and compared to predictions from a thermodynamic model. Both the corona block length and the solvent water ethanol ratio were changed, leading to a thorough test of this model. With increasing ethanol fraction, the PEP core-solvent interfacial tension decreases, and the solvent quality for PEO changes. The weight-average block masses were 5.0 kDa for PEP and 2.8-49 kDa for PEO. For the lowest PEO molar mass and samples in pure water (except for the highest PEO molar mass), the micelles were cylindrical; for other conditions they were spherical. The structural parameters can be reasonably well described by the thermodynamic model by Zhulina et al. [Macromolecules 2005, 38 (12), 5330-5351]; however, they have a stronger dependence on solvent composition and PEO molar mass than predicted.