Coke deposition during toluene alkylation with methanol over a Mg-modified ZSM-5 catalyst leads to drastic changes in the zeolite shape selectivity with the time-on-stream. Besides a decrease in toluene and methanol conversions, a maximum in para selectivity is observed due to coke deposition. Nevertheless, the deactivated catalysts are less para-selective than the fresh ZSM-5 when they are compared at the same toluene conversion. Para selectivity of the primary product decreases due to coking below the value of 100% corresponding to the uncoked catalyst. At low times-on-stream, coke deposition takes place mainly by pore filling. At higher times-on-stream, the proportion of polyaromatic species in the coke deposits is increased, which leads to a partial blockage of the zeolite pore structure, lowering the length of the diffusional pathway. Therefore, the intracrystalline diffusional control on the internal xylene isomerization is less pronounced in the coked catalysts, which causes the lower para selectivity of the primary product observed in these samples.