Alkyl imidazolium-functionalized poly(ether ketone)s (Im-PEKs) were prepared as novel polymer membranes for O2/N2 and CO2/N2 separations. The Im-PEK polymers were synthesized by polycondensation between difluoro- and dihydroxy-monomers, followed by benzylic bromination and alkyl imidazolium-functionalization. The polymers with pendant imidazolium-based ionic salts showed high CO2 solubility and hence a very high CO2/N2 permselectivity of 66.1 due to their high solubility selectivity. In addition, the Im-PEK with long alkyl chain ([C12-Im-PEK][Br]) displayed an extraordinary high O2/N2 permselectivity of 15.5 due to its enhanced diffusivity selectivity. Overall, the combined O2/N2 and CO2/N2 separation properties were achieved in single polymeric membrane. The Im-PEK membranes also displayed good thermal and mechanical stabilities.