The association of a block copolymer polystyrene-b-poly(ethylene/propylene) in mixtures of 5-methyl-2-hexanone (a selective solvent of polystyrene block) and 2-pentanol (a precipitant of both copolymer blocks) has been studied by static and dynamic light scattering and viscometry. Micelle structural parameters have been analyzed as a function of the solvent composition. As the precipitant percentage increases in the solvent mixture, the molar mass increases, the apparent radius of gyration and the second virial coefficient decrease, and the hydrodynamic radius remains constant. Static light scattering was used to establish the dependence of the critical micelle temperature on concentration. Dynamic light scattering measurements have shown that the micelles have narrow size distributions and micellization can be thermodynamically treated as a closed association. The light scattering results were used to calculate the standard Gibbs energies, enthalpies, and entropies of micellization. As the 2-pentanol percentage increases, the standard Gibbs energy becomes more negative, whereas the standard enthalpy and entropy remain almost constant.