The La2O3 and CeO2 modifications of H-ZSM-5 zeolites were examined to improve the shape-selectivity in the alkylation of mononuclear aromatic hydrocarbons and related reactions. The selectivities of p-diethylbenzene (p-DEB) among diethylbenzene isomers were improved without significant loss of catalytic activity by the modification with these oxides in the ethylation of ethylbenzene (EB). The La2O3 modification enhanced the selectivities of p-DEB more effectively than the CeO2 modification. The improvement of the selectivities over these oxides is due to the prevention of the isomerization of p-DEB at external acid sites. The para-selectivity for La2O3-modified H-ZSM-5 zeolites is improved due to "product selectivity" resulting from the preferential diffusion of p-DEB by the adjustment of pore entrance as well as by the deactivation of external acid sites. The enhancement of the formation of less bulky xylene isomer was also observed in the disproportionation of toluene and the isomerization of xylene isomers over La2O3- and CeO2-modified H-ZSM-5 zeolites. (c) 2005 Elsevier B.V. All rights reserved.