When polymer blends are foamed by physical foaming agents, such as CO2 or N-2, not only the morphology and viscosity of the blend polymers but also the solubility and diffusivity of the physical foaming agents in the polymers determine the cellular structure: closed cell or open cell and monomodal or bimodal. The foam of poly(ethylene glycol) (PEG)/polystyrene (PS) blends shows a unique bimodal (large and small) cellular structure, in which the large-size cells embrace a PEG particle. Depending on the foaming condition, the average size of the large cells ranges from 40 to 500 mu m, whereas that of small cells becomes less than 20 mu m, which is smaller than that of neat PS foams. The formation mechanism of the cellular structure has been investigated from the viewpoint of the morphology and viscosity of the blend polymer and the mass-transfer rate of the physical foaming agent in each polymer phase. The solubility and diffusivity of CO2, which determine the mass-transfer rate of CO2 from the matrix to the bubbles, were measured by a gravimetric measurement, that is, a magnetic suspension balance. The solubility and diffusivity of CO2 in PS differed from those in PEG: the diffusion coefficient of CO2 in PEG at 110 degrees C was 3.36 X 10(-9) m(2)/s, and that in PS was 2.38 X 10(-10) m(2)/s. Henry's constant in PEG was 5600 cm(3) (STP)/(kg MPa) at 110 degrees C, and that in PS was 3100 cm(3) (STP)/(kg MPa). These differences in the transport properties, morphology of the blend, and CO2-induced viscosity depression are the control factors for creating the unique cellular structure in PEG/PS blends. (C) 2005 Wiley Periodicals, Inc.