The "molecular sieving effect" of zeolites has enabled size-selective adsorption and catalysis. Although a large variety of zeolites have been developed thus far, it is still challenging to find zeolites that can separate molecules such as CO2, N-2, CH4, and small organics that have kinetic diameters all closely located in the range of 0.3 to 0.4 nm. Here we demonstrate that controlled collapse or atomic disordering of NaA zeolite can systematically narrow the effective pore size below 0.4 nm and thus "tune" the molecular sieving effect. As the zeolite is gradually disordered, the adsorption amounts for all gas molecules decrease; however, larger molecules show a much faster decrease than that of the smaller ones. Consequently, the adsorption selectivities could be remarkably enhanced for various gas pairs.