Branched and cyclic hydrocarbons such as iso-butane, 2-methylpentane, 3-methylpentane, 2,2-dimethyl-butane and benzene are preferentially adsorbed at the intersections of the channels of MFI zeolite; they serve as bottlenecks for molecular traffic in mixtures with linear C1-C6 alkanes. Molecular dynamics simulations show that as the loadings of tardier iC4, 2MP, 3MP, 22DMB, and Bz is progressively increased to four molecules per unit cell the diffusivity of the more mobile linear alkane reduces nearly to zero. The reduction in the n-alkane diffusivity is quantitatively similar irrespective of the branched/cyclic hydrocarbon. (C) 2007 Elsevier B.V. All rights reserved.