Results of Molecular Dynamics simulations of tracer exchange of guest molecules between zeolite crystals and the surrounding gas phase are reported. The crystal surface is assumed to be free of structural defects that may additionally slow down molecular transport. The main focus of this study is on the influence of the potential energy difference between the intra- and intercrystalline space of the crystals on adsorption and desorption. Our results illustrate that such energy difference may be responsible for the appearance of flat (i.e. adsorption- or desorption-limited) intracrystalline concentration profiles for sufficiently small zeolite nanocrystals present in novel hybrid materials. (c) 2006 Elsevier B.V. All rights reserved.