A simple parameter is proposed for comparing the performance of two or more adsorbents for a particular binary gas separation by pressure swing adsorption. The parameter is most suitable for separations based on differences in equilibrium adsorption capacity rather than on differences in adsorption kinetics. The two main components of the parameter are the ratio of the delta loadings (differences in adsorbed amounts at high and low pressures) of the 2 gases (Deltaq(1)/Deltaq(2)) and the equilibrium selectivity (alpha(1,2)) of the sorbent for the strongly adsorbed species. A dimensionless sorbent selection parameter (S) is defined. For a given separation, the sorbent that yields the highest S value is the best sorbent. In the cases of sorbate-sorbent systems showing Langmuir-type isotherms, S = Deltaq(1)/Deltaq(2) (q(s1)b(1)/q(s2)b(2)) This parameter is simple to calculate and is sensitive to subtle differences in adsorption isotherms. The effectiveness of the proposed parameter was demonstrated through two examples of air separation using molecular-sieve zeolites.