The size of pore apertures of zeolite A was systematically controlled by silica deposition on the external surface of the zeolite A from a dilute solution of tetraethyl orthosilicate (TEOS) in dry toluene. The silica deposition on the external surface is confirmed by EDX analysis and external surface area measurements. The adsorption properties of these pore-size-engineered zeolites show that these adsorbents are useful for the size-selective separation of molecules of size in the range of 3-4 angstrom, such as N-2/O-2/Ar and H2O/CH3OH/C2H5OH. Argon and nitrogen molecules are restricted from entering the zeolite pores after depositing TEOS on the external surface of zeolite A, and their adsorption capacities decrease. Comparatively smaller oxygen molecules can still enter the pores and become adsorbed there. O-2/N-2 and O-2/Ar selectivity increase with increase in the TEOS deposition. The adsorption properties of the pore-size-engineered zeolite A become similar to that of commercially available molecular sieve 3A after depositing around 0.20% TEOS on the external surface of NaA.