Polystyrene/poly(2-vinyl pyridine) heteroarm star copolymer (i.e. star-shaped polymers consisting of a central poly(divinyl benzene) core bearing an equal number of polystyrene and poly(2-vinyl pyridine) arms, PS(6)P2VP(6)) and the corresponding diblock copolymer were investigated in toluene, which is a selective solvent for PS. The star copolymers are associated in polymolecular micelles following the closed association model. The characteristics of micelles were determined by means of static light scattering, viscometry and cryo-transmission electron microscopy. The micelles adopt a core-shell structure of spherical shape with core radius almost equal to the corona thickness. Comparing their solution behaviour with that of the corresponding diblock copolymer, having the same block lengths as those of the star arms, significant differences were observed. The A(n)B(n) exhibit cmc which is three orders of magnitude higher and an aggregation number which is about one order of magnitude lower with respect to the AB copolymer. The thermodynamics of micellization were also studied. Delta G(0), Delta H-0 and Delta S-0 were found to be negative for both systems. Delta G(0) is less negative for the star copolymer implying a smaller driving force for micellization. This is due to a pronounced loss of the combinatorial entropy of the star polymer arms.