Reactions of m-xylene are performed over a series of unidimensional, high-silica zeolites. It is determined that zeolites with a unidimensional pore structure have a unique reaction selectivity when compared to multidimensional zeolites of the same pore site. Unlike multidimensional zeolites with 12 member ring (MR) pores that give a paralortho p/o selectivity of >1, zeolites with unidimensional pores bounded by 14 MRs or large 12 MRs give a plo <1 due to the influence of a bimolecular isomerization mechanism between a xylene and trimethybenzene. The unidimensional, parallel pore zeolites (UTD-1, SSZ-31, CIT-5, SSZ-24, ZSM-12, ZSM-48) gave products with the lowest plo ratio for the given pore size. Multidimensional zeolites and unidimensional zeolites with internal cavities larger than the pore openings all gave higher plo selectivities. It is shown that at lower flow rates, where external diffusion becomes important, plo selectivity can be lowered due to an increased amount of bimolecular isomerization occurring at or near the catalyst external surface. While the selectivity of the reactions of In-xylene give useful information for characterizing zeolite structures, it is imperative that all zeolites be compared at the same flow rate and that the flow rate be sufficiently high to negate significant external diffusion effects on reaction selectivity. Under the proper conditions, the reactions of m-xylene can give information enabling the characterization of medium, large and extra-large parr zeolites.