Ceria-modification is an effective method for the deactivation of external acid sites of H-mordenite (HM). The selectivity for 4,4'-diisopropylbiphenyl (4,4/-DIPB) in the isopropylation of BP over dealuminated HM such as HM(128) decreased with raising reaction temperatures or with decreasing propylene pressures, The decrease of the selectivity during the isopropylation is due to the isomerization of 4,4'-DIPB. Ceria-modification of HM(128) was highly effective for the prevention of the isomerization. The selectivity for 4,4'-DIPB was also improved in the case of HM with low SiO2/Al2O3 ratio such as HM(10). The enhancement of the selectivities of 4,4'-DIPB by ceria-modification is ascribed to the decrease of external acid sites which are active in non-regioselective alkylation and isomerization of products.