Permeability of gas molecules H-2, O-2, N-2, and CO in four types of ionic liquids ([C-4-Mim][NTf2], [C-10-mim] [NTf2], [Ng(8881)][NTf2],[C8Py] [NTf2]) with a common anion but different cations supported on nanotiltration membranes was studied in single gas feed and under varying gas phase pressure. The separation property in terms of H-2/CO selectivity in the selected ionic liquids was examined in binary gas feed system and also under varying pressure. The adoption of supported ionic liquid membrane (SILM) using nanofiltration (NF) membrane as support is entirely new and its operating stability was investigated aiming to rectify the inherent instability problem associated with normal supported liquid membranes (SLMs) using conventional solvents residing within microporous support structure such as microfiltration membranes. Gas permeation rate through the new SILM exhibited an exponential increase with increasing gas phase pressure in the range from 3.0 to 7.0 bar, and the SILMs based on the NF membrane were remarkably stable even at the high pressure which far exceeded the pressure range for normal SLMs (< 0.1 bar). H-2/CO separation factor, up to a maximum value of 4.3, exhibited variation among the ionic liquids, and more interestingly, a nonlinear pattern of decline with increasing gas phase pressure in all four ionic liquids, reversing the trend of exponential increase of permeation rate. Further analysis were carried out in an attempt to relate the gas transport and separation properties to physic-chemical properties of the ionic liquids such as viscosity, polarity, and variation in cation molecular structure. The new SILMs supported on nanofiltration membrane and its good stability at operating pressures relevant to industrial applications opens up new route for exploration of supported liquid membrane technology for gas enrichment, extraction and separation. (c) 2006 Elsevier B.V. All rights reserved.