A perturbational approach is used to study the adsorption behavior of small molecules in faujasite type zeolites. From the evaluated interaction energies. iso-interaction energy plots for N-2, O-2, CO2 and C2H4 in LSX faujasite exchanged with Na+ are calculated. From these plots the preferential adsorption sites in the zeolite cavity were located. The interaction energies on the preferential sites were found to predict rough trends for the separation behavior of the considered zeolites. A study of the different cation sites. based on an ab initio calculation of the atomic charges at the HF/3-21G level showed a good agreement with existing experimental results and previous theoretical calculations. (C) 2002 Elsevier Science B.V. All rights reserved.