We present a new theoretical method to study the encapsulation of small molecules such as H-2, O-2, N-2, Ar and CH4 in the CS3Na9-A zeolite. To study the properties of encapsulated molecules, we used the Fermi-Dirac like statistics. The density of states, the distribution function, average binding energy and the average activation energy of encapsulated molecules are calculated. As the number of encapsulated molecules in zeolite cavities increases, the higher energy states in the cavities are gradually filled and, consequently, the activation energy for decapsulation is lowered. We also calculated the fraction of molecules with higher energy than their activation energy, revealing that the activation energy for decapsulation depends not only on the temperature but also on the number of the encapsulated molecules. (C) 2007 Published by Elsevier B.V.