A series of poly(n-butyl methaerylate)-block-poly(2-(acetoacetoxy)ethyl methacrylate)s (PBMA-b-PAEMA) were Studied according to the formation of core-shell micelles in dilute cyclohexane solution (PBMA -> shell, PAEMA -> core). Although being a strongly segregating system, the appearance of different shapes of micelles Could not appropriately be described by simple space-filling arguments within the geometric model of Antonietti and Forster et al. [J. Chem. Phys. 1996, 104, 9956-9970]. The model was refined through introduction of the aspect ratio of the core-forming monomer unit for the calculation of geometric packing parameters.