Using the methods of potentiometry, UV spectroscopy, and static and dynamic light scattering, the interaction of micelles of a cationic amphiphilic diblock copolymer with an oppositely charged linear homopolyelectrolyte in dilute aqueous solutions is studied. The copolymer is represented by polystyrene-block-poly(N-ethyl-4-vinylpyridinium bromide); the polyelectrolyte, by poly(methacrylic acid) and its sodium salt, sodium poly(methacrylate). As is shown, polystyrene-block-poly(N-ethyl-4-vinylpyridinium bromide) in water forms spherical micelles with the core composed of insoluble polystyrene units and the corona composed of cationic blocks of poly(N-ethyl-4-vinylpyridinium bromide); the average aggregation number is about 40. The addition of poly(methacrylic acid) or sodium polymethacrylate is accompanied by cooperative electrostatic binding of oppositely charged units, which leads to the formation of interpolyelectrolyte complexes. Depending on the charge ratio in the solution, either water-soluble interpolyelectrolyte complexes (both positively and negatively charged) or an insoluble interpolyelectrolyte complex is formed. The particles of soluble interpolyelectrolyte complexes are also spherical micelles that contain an excess of positively or negatively charged units of the corresponding polyions. The formation of cooperative salt bonds between the oppositely charged polyions takes place inside the micellar corona of the block copolymer and does not affect the shape-determining polystyrene core. A particle of the interpolyelectrolyte complex contains an insoluble polystyrene core, an intermediate water-insoluble layer composed of polyelectrolyte units, and an outer lyophilizing layer (corona) composed of charged fragments of the excess polyelectrolyte.