Mixed-matrix membranes (MMMs) for CO2/N-2 separation were prepared by incorporating nickel-substituted ZIF-8 (Zn/Ni-ZIF-8) into a polyether-block-amide (PEBA2533) polymer, in which the Zn and Ni ions in Zn/NiZIF-8 have different affinities for CO2 based on p-complexation; these MMMs exhibited exceptional molecular sieving properties compared to ZIF-8-PEBA MMMs. Scanning electron microscopy revealed that Zn/Ni-ZIF-8 nanoparticles were small and uniformly distributed within the PEBA, and attenuated total reflectance Fourier-transform infrared spectroscopy revealed good interfacial compatibility between the Zn/Ni-ZIF-8 and PEBA; consequently these MMMs exhibited excellent mechanical properties. The 10% Zn/Ni-ZIF-8-PEBA MMM showed a high CO2 permeability of 321 Barrer, and a CO2/N-2 selectivity of 42.8 at 2 bar; these values were much higher than those of the 10% ZIF-8-PEBA MMM (266 Barrer and 33.8, respectively). We assume that different metal-ion affinities towards CO2 resulted in the asymmetric adsorption/desorption behaviour of these MMMs, which led to a significantly higher permselectivity for the Zn/Ni-ZIF-8-PEBA MMM. The MMM loaded with 10% Zn/Ni-ZIF-8 exhibited long-term CO2-separation-performance stability under mixed-gas conditions, with a CO2 permeability of 282 Barrer and a CO2/N-2 selectivity of 42.7. Moreover, the permselectivity of the 10% Zn/Ni-ZIF-8-PEBA MMM under pure-gas condition surpassed Robeson's upper bound reported in 2008.