A comparison of the free-energy contour maps for systems involving a model dealuminated H-mordenite surface zeolite and three different hydrocarbons ((a) n-hexane, (b) isooctane, and (c) 2-methylpentane) is made on the basis of Monte-Carlo simulations. The computed isosteric heats of adsorption at infinite dilution and at 298 K, respectively (a) 55.0 +/- 2.0 kJ . mol(-1), (b) : 105.5 +/- 5.0 kJ . mol(-1), and (c) 34.5 +/- 2.0 kJ . mol(-1), are in fair agreement with the available experimental data and previous simulations. Attention was focused mainly on the interface region and surface properties were specifically investigated. A detailed analysis of the results allows a clear description of the geometrical features of the trajectory followed by the molecules when entering a pore. Along with the molecular size, the internal flexibility of the adsorbate is a key parameter to understand both these features and the energy of the adsorption : Linear molecules, enjoying maximal flexibility, are preferred to their branched isomers. In this concern, it is clear that the adsorbate’s ability to distort, along with its global size, has also to be taken into account in the notion of molecular sieving by zeolites.