To gain a deeper insight into shape selectivity on zeolites, ethylbenzene disproportionation was studied in detail by computer simulation and experiment. Two continuum models were developed : first a microkinetic (pseudo homogeneous) one, which considers only intrinsic chemical kinetics and can be solved analytically. Second, a more complex macrokinetic (heterogeneous) one, which additionally considers transport phenomena and can be solved only numerically. Both models allow a study of the dependence of shape selectivity on several parameters. Such important parameters include the activity and diffusivity of the reaction system and the operating conditions of the gradientless recycle reactor, which was chosen for simulation and experiment, respectively. For the experimental investigations of shape selectivity in ethylbenzene disproportionation a HZSM-5 zeolite was used. Geometric constraints on the product molecules within the pores of HZSM-5 zeolite favour the formation of para-diethylbenzene. Therefore, shape selectivity causes high para-selectivity. Comparing experimental data to simulated results, the microkinetic model allowed an adequate Bt of para-selectivities which points out that the corresponding assumptions seem to be true. Thus it is concluded that under the chosen experimental conditions para-isomer is exclusively-formed inside the pore structure and subsequent isomerization occurs on the external surface only.