The modification of H-mordenite (MOR) with lanthanide oxides La2O3, CeO2, Pr2O3, Sm2O3, Dy2O3, and Yb2O3 was examined for the deactivation of external acid sites and confirmed in the cracking of 1,3,5-triisopropylbenzene (TIPB) and cumene (IPB) and in the isopropylation of biphenyl (BP). The cracking of TIPB, which cannot enter the pores of MOR, shows that external acid sites were effectively deactivated by the modification of MOR with the lanthanide oxides in small amounts. Only the cracking of IPB over CeO2-modified MOR exhibited excellent catalytic activities, even at a 30 wt % metal loading, whereas the activities of other lanthanide oxide-modified MOPS rapidly decreased as the loadings were increased because pore entrances became choked. The isomerization of 4,4'-diisopropylbiphenyl (4,4'-DIPB) during the isopropylation of BP at high temperatures such as 300 degrees C was also effectively prevented by the modification of MOR with the lanthanide oxides. Particularly, CeO2-modified MOR remained highly active even at a 30 wt % loading. Other lanthanide oxides can deactivate the isomerization of 4,4'-DIPB at 5-10 wt % loadings without significant loss of the activities at 300 degrees C, while the activity was rapidly lost as the loading amount was increased. The physicochemical properties of lanthanide oxide-modified MORs indicate that the lanthanide oxides modify the surface properties of MOR The amounts of N-2, o-xylene, and NH3 adsorbed on MOPS mostly remained high after CeO2 modification; however, they rapidly decreased when loadings of the other oxides increased. These results show that CeO2 remains the open pores at high loadings; however, the other oxides reduce the size of pore entrances as the loading is increased.