The shape selectivity of H-ZSM-5 in the disproportionation of ethylbenzene was investigated, using a set of samples with the same Si/Al ratio, but different particle sizes in the range from 0.1 to 80 mu m. The number of external acid sites of each sample was measured gravimetrically by the adsorption of 2,6-dimethylpyridine. The data were correlated with the results of catalytic experiments. Conversion and product distribution are strongly dependent on the external acidity which in turn correlates well with the particle size. An estimate for the diffusion coefficient could be obtained by fitting the effectiveness factor for the different particle sizes.