AbstractThe larger pore LaY zeolite catalyst resembles homogeneous metal chloride and heterogeneous silica‐alumina acid catalysts since the results are consistent with isomerization being a series of intramolecular 1,2 shifts. On the other hand, isomerization with the smaller pore zeolite, ZSM‐5, is very different and the relative rate constants appear to be influenced by diffusion and/or shape selectivity. Disproportionation is a significant reaction with LaY but not with ZSM‐5 in the temperature range used for the present study. The small amount of trimethylbenzene formed at low conversions using ZSM‐5 is 1,2,4‐trimethylbenzene, also providing evidence of shape selectivity influencing the reaction