Supported ionic liquid membranes (SILMs) based on [EMIM] [Ac] and gamma-Al2O3 membranes were prepared using a supercritical fluid deposition (SCFD) method at 12 MPa and 50 degrees C. The influences of the preparation time, ionic liquid loading, and ethanol content on the performance of SILMs were studied. The CO2/N-2 selectivity of the as-prepared SILMs reached the ideal selectivity of [EMIM][Ac]. When the ionic liquid (IL) was dissolved in a mixture of supercritical CO2 (scCO(2)) and ethanol, the IL easily diffused into the small pores in supports. Moreover, the dissolved IL could transition to the liquid phase in the small pores because of the capillary phase transition, leading to the selective filling of the meso- and micropores in the supports. The surface tension of the liquid phase and the ratio of the IL concentration to the saturation IL concentration were found to be the key factors influencing the capillary phase transition of the IL.