Conditions of the polymolecular micellization of a linear three-block copolymer-polystyrene-block-polybutadiene-block-polystyrene in a binary solvent tetrahydrofuran-ethanol were determined. Photochemical stabilization of micelles by the crosslinking of polybutadiene chains in the cores induced by intense UV radiation was carried out in a tetrahydrofuran-ethanol mixture (34.5 vol. %), in which compact micelles with the polybutadiene core are formed and, at 25 °C, the micellization equilibrium is markedly shifted in favour of micelles containing a large number of associated copolymer chains. The kinetics of crosslinking of the micellar cores and the behaviour of stabilized micelles in solution were investigated.