Configurational-bias Monte Carlo simulations of the adsorption isotherm for iso-butane (iC4) in MFI zeolite shows a strong infection at a loading Theta = 4 molecules per unit cell. The consequence of the isotherm inflection on the loading dependence of the Maxwell-Stefan (M-S) diffusivity, D, was investigated using infra-red microscopy (IRM). The experimental data show that D decreases nearly linearly as H is increased to 4; further increase in H results in a sharp increase in D, with a cusp at Theta = 4. Kinetic Monte Carlo (KMC) simulations are used to rationalize the experimental observations. The simulations indicate that significant inter-molecular repulsions are present for Theta > 4. (c) 2008 Elsevier B. V. All rights reserved.