Poly(tert-butyl acrylate)-block-poly(2-vinylpyridine) (PBA-b-PVP) in acidic aqueous solutions forms micelles with PBA cores and PVP shells. When this micellar solution is brought to pH higher than 4.8 , the PVP shells collapse and the copolymer precipitates. However, when a sample of acid water soluble poly(2-vinylpyridine)-block-poly(ethylene oxide) (PVP-b-PEO) is present, its PVP blocks coprecipitate with PVP blocks of PBA-b-PVP and form a dense outer core. The PEO blocks then stabilize this complex onion-type structure. Static and dynamic light scattering measurements revealed that for small values of the (PVP-b-PEO)/(PBA-b-PVP) mass ratio the resulting particles are multimicellar clusters. When the ratio is sufficiently large, the solution contains onion-type micelles. However, the outer core of these micelles can incorporate only a limited number of PVP blocks from the PVP-b-PEO copolymer. The excess PVP-b-PEO molecules then form independent smaller micelles with PVP cores and PEO shells.