The effect of water content in 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) on CO2/N-2 separation performance of polyethersulfone supported ionic liquid membrane has been investigated theoretically and experimentally. A small addition of water in [bmim][BF4] obviously improves the performance of the membrane. CO2 permeance increases from 11.5 to 13.8 GPU and CO2/N-2 selectivity increases from 50 to 60, where the water molar fraction increases from 0 to 0.10 at the cross-membrane pressure difference of 0.24 MPa. The improvement of the CO2 permeance at low water content can be attributed to the increase of CO2 diffusivity due to the decrease of viscosity with increasing the water content. While, the CO2 permeance decreases at high water content because of the decreasing CO2 solubility, which is mainly caused by the hydrogen bond interaction between water and [bmim][BF4]. A corrected solubility coefficient model of CO2 in ionic liquid is proposed to evaluate the influence of water-[bmim][BF4] interaction on CO2 solubility. The comparison of CO2 permeance between theoretical values and experimental ones demonstrates that the number of water molecule bound to a [BF4](-) is between 1 and 2 in the range of the water content in our research. (C) 2010 Elsevier B.V. All rights reserved.