This work evaluates the CO2, CH4 and N2 permeation properties of five novel composite membranes based on polymeric ionic liquids (PILs) having different cation pendant units, namely imidazolium, pyridinium, pyrrolidinium, ammonium and cholinium, combined with the same counter-anion ([NTf2]−). The results show that the CO2 permeability in the composite membranes is related to their respective CO2 diffusivities, which are dependent on the PIL polycation nature. The composite membranes of the PILs containing different polycations and 10 wt% of free ionic liquid (IL) with similar structures to each PIL monomeric unit have CO2 permeabilities between 3.66 and 20.4 Barrer, while they exhibit permselectivities ranging from 28.3 to 43.0 for CO2/CH4 and from 25.8 to 34.7 for CO2/N2. The best CO2 permselectivities were obtained when tetra-alkyl ammonium-based PILs (pyrrolidinium, ammonium and cholinium) are used rather than imidazolium or pyridinium. Also, the results of this work indicate that the polycation backbones of PILs can also play a significant role in the design of PIL–IL membranes with the finest gas permeation properties and improved CO2 separation performances.