Poly(tert-butyl acrylate)-block-poly(2-vinylpyridine) (PBA-b-PVP) in acidic aqueous solutions forms micelles with PEA 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)-bloch-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.