The effects of the acid site densities of MOR zeolites on their product selectivity and deactivation were investigated in the methanol-to-olefin (MTO) reactions. The MOR zeolites showed a high conversion and high yield of the lower olefins at the initial time of the MTO reaction, regardless of their acid site density. the conversion and the yield of the lower olefins over them rapidly decreased in the order of However, MOR(103) < MOR(55) < MOR(12) < MOR(5). where the numbers in parenthesis represent their Si/Al ratios. The materials occluded on the MOR zeolites during the MTO reaction were investigated by the IR, C-13 NMR and UV-vis spectroscopic methods. A large amount of polycyclic aromatic hydrocarbons (PAHs) with 3-4 fused aromatic rings was occluded on the MOR(5) zeolite with the highest acid site density. However, alkylbenzenes and alkylnaphthalenes were the major components of the materials occluded on the MOR(103) zeolite with the lowest acid site density. The IR spectra recorded in situ during the MTO reaction over the MOR zeolites clearly exhibited the differences in the accumulation rates of PAH on their pores according to the acid site density. The rapid accumulation of PAH on the MOR zeolites with high acid site densities leads to the loss of the active alkylbenzene intermediates as well as to the blockage of the pores, resulting in their rapid deactivation. The sparse distribution of acid sites in the pores of the MOR zeolites suppresses the condensation of alkylbenzenes in their pores and lengthens their catalyst lifetime. (c) 2008 Elsevier B.V. All rights reserved.