We report on tunable zeolite-based thin film nanostructures whose molecular size selectivity can be controlled between 3 and 5 Angstrom by fast ion exchange. The composite films are assembled from molecularly attached zeolite CaA crystals and a silica binding phase on the gold electrodes of piezoelectric quartz crystal microbalances (QCM). Intrazeolitic cations can be reversibly exchanged with other cations from aqueous solution, such as Na+, K+, and Rb+. The mass changes resulting from cation exchange in the film can be measured ex situ due to the high stability of the film and the nanogram sensitivity of the piezoelectric crystal. The surface area and microporosity of the films were studied in situ on the piezoelectric crystals by measuring sorption isotherms. Distinctive differences in the nitrogen sorption between the CaA films and the Na, K, and Rb forms are due to blocking of the zeolite eight-ring windows. The effective pore sizes of the films were evaluated by measuring isotherms and sorption rates of vapors with different molecular sizes and shapes. Step by step exclusion of progressively smaller molecules from the interior porosity of the film is observed as a function of the cation size and distribution in the zeolitic framework.