Self-diffusion NMR spectroscopy and relaxometry have been employed to study fragrance encapsulation in water-soluble, amphiphilic star block copolymers. Diffusion coefficients of four different fragrance molecules in the free form and in the presence of the polymer have been determined and used to calculate the effective degree of encapsulation. In dilute aqueous solutions between 65% and 99% of the guest molecules are trapped inside the polymer. The degree of encapsulation depends on the hydrophobicity of the guest molecule, expressed by the octanol/water partitioning coefficient (log Pow), where high log />ow molecules are nearly quantitatively dissolved in the polymer. The fragrance molecules are mainly located in the hydrophobic core of the polymer, which is tightly packed, whereas the hydrophilic shell is flexible and takes up only a small percentage. Proton longitudinal (T1) and transverse (T2) relaxation times of the fragrance molecules are significantly reduced in the presence of the polymer indicating slower rotational correlation times due to microsolubilization in the hydrophobic core. © 2007 American Chemical Society.