The hydroisomerization of n-hexane on large crystals (50 µm) of Pt-loaded H–mordenite has been used as a test reaction to study the effects of concentration-dependent diffusion on zeolite catalyzed reactions. This concentration dependence was observed in the form of a nonlinear deviation from the relation between the reaction rate and the n-hexane pressure as provided by the intrinsic reaction rate equation. The dependence of the activity on the n-hexane pressure was measured at various temperatures and the results were compared with the results of model calculations. In the model used for these simulations it was assumed that the effective diffusion coefficient was proportional to (1-¿)/¿(¿ = fraction of occupied sites) as proposed by K. Hahn and J. Kärger (J. Phys. Chem. B102, 5766 (1998)) and P. H. Nelson and S. M. Auerbach (J. Chem. Phys.110, 9235 (1999)) for single-file diffusion at long times. Furthermore, since it is thought that immobile alkoxy intermediates are present in the pores under reaction conditions, it was assumed that the effective diffusion coefficient was proportional to the fraction of surface species consisting of hexanes and hexenes. Except for the effective single-particle diffusion coefficient, for which no reliable literature value was found, independently obtained input parameters were used in the model calculations. Good agreement between experiment and model was obtained using a value for the effective single-particle diffusion coefficient of 10-5 m2/s, which is well within the range of orders observed for the single-particle diffusion coefficients of methane and tetrafluormethane in molecular sieves.