The interaction of polyisobutylene-block-poly(sodium methacrylate) diblock copolymer micelles with a strong cationic polyelectrolyte, poly(N-ethyl-4-vinylpyridinium bromide), in alkaline media was examined by means of turbidimetry, analytical ultracentrifugation, and fluorescence spectroscopy. It was shown that the diblock copolymer micelles and the cationic polyelectrolyte, taken at charge ratios Z = [+]/[-] not exceeding a certain critical value Z(M) < 1, form peculiar water-soluble micellar complex species, each containing a two-phase hydrophobic nucleus and a hydrophilic corona. The nucleus consists of a polyisobutylene core and a shell assembled from the fragments of the water-insoluble interpolyelectrolyte complex. The corona is formed by the excess fragments of poly(sodium methacrylate) blocks not involved in the complexation with poly(N-ethyl-4-vinylpyridinium bromide).