The ethylation and the disproportionation of ethylbenzene on ZSM-5 zeolites yield a mixture of the three isomers ortho-, meta-and para-diethylbenzene. The two reaction systems were simulated on the basis of an adsorption/diffusion/reaction-model using both a continuum and a Monte Carlo approach. Para-selectivity, i.e. the fraction of the para-isomer in the isomer mixture, depended on several facto rs : the Thiele modulus of ethylation/disproportionation, the primary product distribution, and the relative diffusivity of diethylbenzene isomers. The simulation results obtained by both methods were in good agreement. The nature of the shape selectivity could be discussed in detail. By comparing experimental results from literature with these simulations, it was concluded that in the ethylation para-selectivity can be mainly attributed to product rather than transition state shape selectivity. On the contrary, for the disproportionation it is mon reasonable to ascribe the observed para-selectivity to transition state shape selectivity. On the basis of the simulations, some suggestions to enhance the para-selectivity for different cases were proposed.