Colloidal platinum (diameter 1-2 nm) in styrene was prepared in situ by reduction of a platinum(II) compound followed by partial evaporation of styrene, The resulting dispersions were purified from the reaction side-products, and ammonium O,O ' -dialk-yldithiophosphates of different chain length or octadecanethiol was added. Polymerization was started with a radical initiator. Several parameters were varied and, particularly in the presence of long-chain dialkyldithiophosphates, self-assembled superstructures of metal colloids appeared at certain conditions in the resulting poly(styrene)-platinum nanocomposites. Unusual hollow shell structures of typical diameters of 50-300 nm were found, which phenomenologically resembled bilayer vesicles in aqueous solutions, although the formation mechanism of the bilayers and the superstructures in the nanocomposites is considered to differ. It is supposed that the formation of the superstructures in the nanocomposites is induced by crystallization of alkyl chains adsorbed at the platinum surface below monolayer coverage.