Abstract The introduction of suitable CO2-philic additives, enabling high CO2 concentration retained in the polymer matrix, might be a promising strategy to obtain foams with small cell size and large volume expansion ratio. The effect of molecular weight (Mn) (2000–17000 g/mol) of CO2-philic additives (PDMS and PVAc) on the diffusion of supercritical CO2 into and out of polystyrene (PS) was investigated via experiments and dissipative particle dynamics simulations. The CO2 diffusion coefficient increased with the presence of additives especially with PDMS (2000 g/mol, from 1.2 × 10−9 m2/s to 2.8 × 10−9 m2/s). Meanwhile, the saturation solubility of CO2 increase with the incorporation additives. Compared with high Mn additives, the slower CO2 desorption rate with additives with Mn of 2000 g/mol enabled higher amount of CO2 retained in PS, resulting in higher cell density. The incorporation of PDMS (2000 g/mol) decreased the cell size (from 11.67 to 7.57 μ m) and increased volume expansion ratio of PS foams.