The He(Ne)/NaA-zeolite system was studied using temperature programmed desorption mass spectrometry (TPD-MS) with a supersonic molecular-beam inlet. Controllable, stable, and reversible entrapment of He and Ne by the beta cages of NaA zeolite was experimentally achieved at ambient pressure and around 200 degreesC. Decapsulation of either He or Ne from NaA is shown to be of a doublet character, indicating on the occurrence of effectively two classes of beta cavities: completely blocked cages, never previously observed, and partly blocked ones. The encapsulation of Ne and He in NaA is associated with the coupling of two reversible mechanisms governing the effective free aperture dimension, i.e., apertures thermal dilation and activated ion mobility. Characteristic admission temperatures between 130 degreesC and 200 degreesC, show highly selective sieving effect between He and Ne, suggesting its potential utilization for gas separation via a temperature swing practice and for a possible experimental realization of quantum sieving. (C) 2003 American Institute of Physics.