Room Temperature Ionic Liquids (RTILs) based on the 1-n-alkyl-3-methylimidazolium cation were immobilised in polymeric membranes, in order to study the potential of using supported ionic liquid membranes (SILMs) for CO(2)/N(2) and CO(2)/CH(4) gas separations. Different aspects were investigated, namely: the evaluation of the SILMs stability using two membrane supports which differ in terms of their hydrophobicity; and the effect of using RTILs with cations of different alkyl chains and also with different anions, on the permeability and selectivity of pure and humidified pure gases as well as gas mixtures. H(2), O(2), N(2), CH(4) and CO(2) gas permeabilities were determined and CO(2)/N(2) and CO(2)/CH(4) ideal selectivities were calculated and compared with data available in the literature described by the Robeson upper bound correlation. The effect of the presence of water vapour in different gas streams of N(2), CH(4) and CO(2) was also studied. Finally, CO(2)/N(2) and CO(2)/CH(4) binary mixtures (50%, v/v) were prepared and the selectivity obtained was compared with the ideal selectivity. The results show that the SILMs prepared with the most hydrophobic support are more stable than those based on the hydrophilic support, and have a high affinity for CO(2) when compared with other gases. This behaviour was observed both for pure and gas mixtures, at low pressures. The high selectivity values obtained for CO(2) indicate that these SILMs may be considered for CO(2) separation processes. (C) 2010 Elsevier B.V. All rights reserved.