The self-assembly behavior of cationic block copolymers of polystyrene-block-poly(N-ethyl-4-vinylpyridinium bromide) (PS-b-PE4VP) with the constant length of PS block (100 units) and varying lengths of ionic PE4VP block (100-500 units) in aqueous dispersions was studied by fluorescence spectroscopy and sedimentation velocity techniques. The values of cmc for PS-b-PE4VP copolymers were within the range 10(-7)-10(-6) M and only slightly increased as the ionic chain length increased. At the concentrations above cmc, a coexistence of the micelles and micellar clusters formed by PS-b-PE4VP copolymers was observed. The distribution of copolymer chains between the micelles and micellar clusters was found to be independent of copolymer concentration and length of ionic PE4VP block but dependent on the concentration of the elementary salt in the solution. Interaction of PS-b-PE4VP micellar aggregates with the oppositely charged surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT), was studied. In contrast to homopolyelectrolyte-surfactant systems that form only stoichiometric insoluble complexes, the formation of insoluble nonstoichiometric and stoichiometric PS-b-PE4VP/AOT complexes was observed. The composition of insoluble PS-b-PE4VP/AOT complexes strongly depends on the length of ionic PE4VP block. Such a difference in behavior of block copolymer-based complexes has been attributed to the presence of block copolymer micelles upon interaction with surfactant ions compared to the individual polymer chains that interact with the surfactant in the case of ionic homopolymer.