The thermodynamics of the micellization process of polystyrene-block-poly(ethylene/propylene) in solutions of polystyrene in toluene was studied. Toluene is a good solvent for both copolymer blocks, whereas polystyrene and poly(ethylene/propylene) are not compatible polymers. The influence ofthe polystyrene concentration on the standard Gibbs energy, Delta G degrees, enthalpy, Delta H degrees, and entropy, Delta S degrees, of micellization was investigated. The thermodynamic magnitudes were calculated from critical micelle temperature and concentration data. The critical micelle temperatures ofthe different micellar solutions were determined by light scattering measurements. All ofthe standard magnitudes of micellization become more negative as the polystyrene concentration increases, suggesting an increment of medium selectivity.