Port-filling concept was applied to prepare high permselective composite membranes for CO2 separation. The membrane is composed of two polymeric materials: a porous substrate and filling polymers that fills the pore of the substrate. In this study, asymmetric polyacrylonitrile (PAN) membrane was used for a substrate and methoxy poly(ethylene glycol) acrylate (MePEGA, M-w = 454) was used for a photoinitiated graft polymer. The analysis of graft membrane by Fourier transform infrared and attenuated total reflection (FT-IRI FT-IR/ATR), electron spectroscopy for chemical analysis (ESCA) and field emission scanning electron microphotography (FE-SEM) revealed that the graft polymer filled the pores of the substrate. High CO2/N-2 permselectivity was shown by this Fore-filling membrane: carbon dioxide permeation flux, J(CO2) = 5.65 GPU (10(-6) cm(3)(STP/cm(2) s cmHg) and selectivity of CO2 over N-2, J(CO2)/J(N2) = 32.4 at 30 degreesC, measured by the time-lag method. The high permselectivity of the present pore-filling membrane is attributed to the high solubility selectivity due to the affinity of CO2 to PEO segment.