We report on small-angle neutron scattering and cryo-transmission electron microscopy of complexes made from polyelectrolyte-neutral block copolymers and surfactants. Two block copolymer/surfactant systems have been investigated. In the first system, the polyelectrolyte block is negatively charged (poly(sodium acrylate), molecular weight 5 000 g/mol) and the neutral block is a poly(acrylamide) chain of molecular weight 30 000 g/mol. This copolymer is studied in solution in the presence of a cationic surfactant, dodecyltrimethylammonium bromide (DTAB). In the second system, the polyelectrolyte block is positively charged (poly(trimethylammonium ethylacrylate), molecular weight 11 000 g/mol) and the neutral block is again a poly(acrylamide) of molecular weight 30 000 g/mol. This copolymer is studied in solution with an anionic surfactant, sodium dodecyl sulfate (SDS). We show that the diblocks copolymers associate with oppositely charged surfactants into colloidal complexes which have a core−shell microstru...