To determine in which way an organometallic Sn(Me)(3) group, grafted on the pore edge of a model dealuminated H-mordenite zeolite, changes the diffusion process of two selected hydrocarbons (n-hexane and isooctane) into the porous material, free-energy contour maps of the involved potential of mean force have been determined on the basis of Monte Carlo simulations using statistical perturbation theory method. Their analysis lends us to conclude that the penetration of the hydrocarbons inside the zeolite is strongly modified by the grafting. The awaited effective pore-size reduction is confirmed, bur the most striking feature is the appearance of an attractive potential well once the molecules have penetrated into the zeolite pore (-80 +/- 15 kJ mol(-1) for rt-hexane. -180 +/- 30 kJ mol(-1) for isooctane). This may accelerate the initial diffusion of the hydrocarbons toward the solid but also may trap the first incoming molecules and thus reduce the whale diffusion process. In the case of n-hexane, no enhancement of the energy barrier involved in the penetration of the molecule within the solid is found (3 kJ mol(-1)), while for isooctane, a real change occurs (+60 kJ mol(-1)). A detailed analysis of the conformational properties of both the grafted group and the hydrocarbon molecules along the diffusion path is reported. As in the case of the plain H-mordenite, the flexibility of the penetrating molecules appears to play a decisive role.