Earlier kinetic investigations of ethylbenzene disproportionation over medium-pore zeolite catalysts revealed that the para selectivity for the diethylbenzenes produced increases strongly with crystal size. Two models for diethylbenzene isomerization have been offered as a possible explanation for the observed dependence: secondary isomerization on the external surface of the zeolite crystallites (I) and isomerization inside the zeolite pores combined with faster diffusion of para-diethylbenzene (II). The present paper describes additional investigations, aimed at obtaining evidence for one of these models. Thus, in a series of catalytic experiments the effect of the deactivation of the outer crystallite surface of H-ZSM-5 samples on the distribution of the diethylbenzene product was examined. Further, sorption of pure para-, meta-, and ortho-diethylbenzene from the gas phase over H-ZSM-5 was studied by means of in situ infrared spectroscopy. Both types of investigations provided evidence that diethylbenzene isomerization occurs in the interior of H-ZSM-5. Thus, the para-selective features of H-ZSM-5 reflect the interplay of catalytic reaction and mass transfer phenomena. It was additionally shown that para-diethylbenzene and meta-diethylbenzene are reversibly sorbed in H-ZSM-5 in contrast to the ortho isomer, which cannot enter the zeolite pore system in the temperature range examined (359-522 K).