The alkylation reaction of benzene with ethane was studied at 370 degrees C over two Pt-containing MFI catalysts with Si/Al ratios of 15 and 40. The deactivation of the PtH-MFI-15 catalyst was found to be more significant, when compared with the PtH-MFI-40 catalyst, as a result of differences between the two catalysts in the formation of coke. The differing locations of the coke deposition were found to profoundly affect the product selectivity and deactivation behaviour of the two PtH-MFI catalysts. Results from gas sorption and X-ray diffraction experiments showed that coke is preferentially formed towards the centre of crystallites of the PtH-MFI-15 catalyst, as opposed to coke deposition on the outside surface of the PtH-MF1-40 crystallites, subsequently blocking entrance to the zeolite channels. Partial blockage of the internal pore structure of the PtH-MFI-15 catalyst with coke decreased the diffusion length within the PtH-MFI-15 crystallites. The effect of this reduction in the diffusion length within the PtH-MFI-15 crystallites is consistent with the observed decreasing para-selectivity of the diethylbenzene (DEB) isomers with time-on-stream. These findings are in contrast to the typical effect of coking, where, generally, the selectivity of para- isomers would be enhanced with coke deposition. (C) 2010 Elsevier Inc. All rights reserved.