In the past 3 decades, there has been great advancement in the preparation of microcellular thermoplastic polymer foams. However, little attention has been paid to thermoplastic elastomers. In this study, microcellular poly(ethylene-co-octene) (PEOc) rubber foams with a cell density of 2.9 x 10(10) cells/cm(3) and a cell size of 1.9 mu m were successfully prepared with carbon dioxide as the physical blowing agent with a batch foaming process. The microcellular PEOc foams exhibited a well-defined, closed-cell structure, a uniform cell size distribution, and the formation Of unfoamed skin at low foaming temperatures. Their difference from thermoplastic foam was from obvious volume recovery in the atmosphere because of the elasticity of the polymer matrix. We investigated the effect Of the molecular weight on the cell growth process by changing the foaming conditions, and two important effect factors on the cell growth, that is, the polymer matrix modulus/melt viscoelastic properties and gas diffusion coefficient, were assessed. With increasing molecular weight, the matrix modulus and melt viscosity tended to increase, whereas the gas solubility and diffusion coefficient decreased. The increase in the matrix modulus and melt viscosity tended to decrease the cell size and stabilize the cell structure at high foaming temperatures, whereas the increase in the gas diffusion coefficient facilitated cell growth at the beginning and limited cell growth because most of the gas diffused Out Of the polymer matrix during the long foaming times or at high foaming temperatures. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1994-2004, 2010