The effect of solvent composition on the partition coefficient (K-v) of pyrene between the crew cut micelles formed from highly asymmetric block copolymers of polystyrene (500)-b-polyacrylic acid (60) and an H2O/DMF solvent mixture was studied using a fluorescence method. The total number of pyrene molecules incorporated per micelle was also determined as a function of polymer concentration and solvent composition. The relationship between the partition coefficient (K-v) and the mass equilibrium constant (K) was obtained, and the free energy of transfer of pyrene from the solvent mixture to the micelle was calculated in terms of K. The results suggest that a gradual change in the solvent composition, which makes it increasingly incompatible with the solubilizate, may be a good method of loading hydrophobic materials into micelles.