CrO3 particles were synthesized via a thermal reaction at 85 degrees C using the ionic liquid 1-butyl-3-methylimida-zolium tetrafluoroborate (BMIM-BF4). A poly(ethylene oxide) (PEO) composite membrane was prepared for CO2/N-2 separation via a dispersion of fabricated CrO3 particles and BMIM-BF4 by sonication. It was hypothesized that the CrO3 particles increased the solubility of CO2 gas molecules. Furthermore, free imidazolium ions of BMIM-BF4 served as agents for enhancing CO2 transport. The permeance and selectivity of a PEO membrane without CrO3 particles were 11.0 gas permeance units (GPU) and 6.5, respectively, while the maximum CO2 permeance of the composite membrane containing CrO3 was 144 GPU with selectivity of 30. These results indicated that the fabricated CrO3 particles improve the solubility of CO2, leading to high efficiency in CO2/N-2 separation. Thus, the permeance and selectivity increased because of the synergistic effect of the increased CO2 solubility by chromate esters generated from CrO3 and the barrier effect on N-2 molecules due to the CrO3 particles.