Theoretical studies of the behaviour of an adsorption system MFI/n-hexane were performed with a molecular dynamic method. Investigation of influence of isomorphous substitution on diffusion coefficients was the main objective. It was found that the presence of metals other than silicon in the zeolite framework might cause an increase in the diffusion coefficients. However, it is small, independently of the adsorption level. (C) 2007 Elsevier B.V. All rights reserved.