Micellization of the complexes of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) with formic acid (FA) takes place in chloroform, leading to regular aggregates. The resultant aggregates were characterized by dynamic light scattering (DLS), static light scattering (SLS), scanning electron microscopy (SEM), and H-1 NMR. The variation of the average radius of gyration () of the aggregates with the molar ratio of FA to the pyridine units of the copolymer (MR) was used as an important clue for the characterization of the morphologies. It is shown that the aggregates have a morphology of conventional micelles or micelles with a low-density core rather than vesicles, depending on the value of MR. Although the conventional micelles are most commonly obtained aggregates via micellization of a block copolymer in its selective solvent, the micelles with a low-density core (similar to a swollen physical microgel) are novel core-shell nanoparticles, which have been rarely reported and are expected to have a great potential for applications.