Values of relative interaction energy between an isobutane molecule and the surface of the H-mordenite zeolite were obtained. Sixteen tetrahedral sites were taken into account for the surface. Results showed that the adsorption energies were 4 and 8 kJ/mol, for ab initio restricted Hartree-Fock (RHF) method and density functional Becke-Lee-Yang Parr (B3-LYP) approaches, respectively, when C-2-H-2 bond is located in front of H+ atom of H-mordenite surface. The minimum value for the interaction in the adsorption was around 2.25 and 2.50 Angstrom distance from the DFT and ab initio method, respectively. The interaction of isobutane by methyl groups side over the catalytic surface caused a repulsive interaction. Therefore, adsorption of isobutane is a selective process by the H-2 side of the molecule. Calculations were done by employing the 6-31 G** basis set. (C) 2003 Elsevier Science B.V. All rights reserved.