The disproportionation of ethylbenzene (EBz) has been investigated over MCM-22 and its dealuminated samples at low temperature. Dealumination was conducted by steaming followed by acid leaching. The number of acid sites was measured by using pyridine and 2,6-di-tert-butyl-pyridine as probes. 1,1,1,3,3,3-Hexamethyldisilazane (HMDS) was used as a selective poison to external acid sites. Under the reaction conditions of this work, MCM-22 showed unexpectedly high initial activity but deactivated rapidly. This peculiar catalytic behavior of MCM-22 is attributed to the high activity of external acid sites. After poisoning of the external surface, the catalytic activity decreased to a very low level but the rate of deactivation decreased significantly. p-Diethylbenzene among diethylbenzene (DEBz) isomers was found to form within the micropore as the primary product. The m- and o-diethylbenzene were mainly produced by the secondary isomerization of p-isomer on the external surface of MCM-22. The para-selectivity of MCM-22 increased with the degree of dealumination. The improvement in para-selectivity by dealumination can be attributed to the suppression of the secondary isomerization reaction because the dealumination treatment removes preferentially the Bronsted acid sites on the external surface rather than those within the micropores. When the concentration of external acid sites of MCM-22 was reduced to a very low level by poisoning or dealumination, the induction period typical of large pore zeolites was observed.