This work evaluates the possibility to tailor the gas separation properties of elastomeric block co-polyamide membranes by means of room temperature ionic liquid (RTIL), based on the 1-n-alkyl-3-methylimidazolium cation. Polymeric gel membranes were prepared by adding from 20 to 80 wt.% of 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM][CF3SO3] to Pebax (R) 1657 and Pebax (R) 2533. The effect of the ionic liquid on the microstructure, properties and performance of these membranes was discussed. DSC analysis showed a gradual decrease of the melting point of the polyamide (PA) blocks in the gel with a decrease in the corresponding melting enthalpy and a complete disappearance of the crystallinity of the polyether (PE) phase with increasing IL content in the case of Pebax (R) 1657. In Pebax (R) 2533 there is virtually no shift in the peak positions and both crystal phases remain present in the gel, indicating effective phase separation between the crystalline fraction of the polymer and the IL, in contrast with the high compatibility and the good mixing of the polymer and the IL observed in Pebax (R) 1657. In the presence of the ionic liquid, the elastic modulus, break strength and maximum elongation all decrease monotonously for Pebax (R) 2533. For the stiffer Pebax (R) 1657 the break strength and maximum elongation show a maximum at low ionic liquid content and then rapidly decrease at higher IL concentration due to compromise between the plasticizing effect of the IL, enhancing its ductability, and the progressive weakening of the gel due to a reduction of the crystal fraction responsible for the mechanical strength. Gas permeation measurements show that the permeability and selectivity of Pebax (R) 2533 are not notably affected by the addition of IL, whereas Pebax (R) 1657 shows a significant increase in the gas permeability. At the same time the permselectivity of the latter for most gas pairs shows a slight decrease, in accordance with the typical Robeson trade-off behaviour. The combination of permeability and selectivity remains nevertheless interesting for applications involving for instance CO2 removal from exhaust gases. (C) 2012 Elsevier B.V. All rights reserved.