This work explores ionic liquid-based membranes with siloxane functionalized cations using two different approaches: supported ionic liquid membranes (SILMs) and poly(ionic liquid)-ionic liquid (PIL-IL) composite membranes. Their CO 2 , CH 4 and N 2 permeation properties were measured at T = 293 K with a trans-membrane pressure differential of 100 kPa. The thermophysical properties of the synthesized siloxane-functionalized ILs, namely viscosity and density (data in Supporting Information), were also determined. Contrary to what was expected, the gas permeation results show that the SILMs containing siloxanefunctionalized cations have lower CO 2 permeabilities than those of their analogues without the siloxane functionality. The addition of siloxane-based ILs into PILs increases both CO 2 permeability and CO 2 /N 2 permselectivity, although it does not significantly change the CO 2 /CH 4 permselectivity. The prepared membranes present very diverse CO 2 permeabilities, between 57 and 568 Barrer, whilst they show permselectivities varying from 16.8 to 36.8 for CO 2 /N 2 and from 9.8 to 11.5 for CO 2 /CH 4 . As observed for other ILs, superior CO 2 separation performances were obtained when the IL containing [C(CN) 3 ] -is used compared to that having the [NTf 2 ] -anion.