The use of block copolymer micelles as novel nucleating agents for foaming thermoplastics is explored. Diblock copolymers can self-assemble into a large number of spherical micelles, making them promising nucleants for the production of microcellular foams with >10(9) cells/cm(3). Different types of A-B diblock copolymers were added to a polystyrene matrix in low concentrations, and the resulting blends were foamed in a batch process using carbon dioxide as the blowing agent. Polystyrene-b-poly(ethylene propylene) and polystyrene-b-poly(methyl methacrylate) diblocks were not effective as nucleants. Diblocks containing poly(dimethylsiloxane) as the core block showed a bimodal cell size distribution and a small increase in cell concentration. The increased solubility of carbon dioxide in poly(dimethylsiloxane) (PDMS), and the reduced surface tension of PDMS, lowered the minimum work of bubble formation and increased the cell concentration. None of the foams showed an increase in cell concentration proportional to the large number of potential nucleants present. Block copolymer micelles as nucleants were not able to sufficiently lower the work of formation for cell nucleation. They are too small in size, can still agglomerate into larger, nonequilibrium structures, and have large contact angles due to the high surface tension of most polymers.