Stabilization of a room temperature methylimidazolium cation based ionic liquid (IL) into the pores of ceramic nanofiltration (NF) membranes was achieved by means of a novel in situ, pressure assisted imbibition/reaction method. The complete coverage of the nanopores void space by the synthesized IL phase, introduced an additional parameter that was pursued in parallel to the challenge of stabilization. The purpose was to develop hybrid membranes that will exhibit solely the gas solvation/diffusion properties of the IL A silane functionalized ionic liquid phase of 1-methyl-3-(1-trialkoxysilylalkyl) imidazolium as cation and hexafluorophosphate as anion, was first synthesized and subsequently grafted onto the pore surface. The CO2/CO separation performance of the modified membranes was quite significant. The maximum CO2/CO separation factors achieved were 11 at 30 degrees C, 25 at 50 degrees C, 12 at 100 degrees C and Sat 230 degrees C. The respective CO2 permeability values were 226, 520, 715 and 2000 Barrer. The stability of the developed membranes has been verified, in 3 sequential cycles of heating/cooling, in the temperature range between 25 and 250 degrees C and under a differential pressure up to 0.5 MPa. (C) 2010 Elsevier B.V. All rights reserved.