The roles of internal and external acid sites of a dealuminated H-mordenite (MOR; SiO2/Al2O3 = 128) on the selectivity for 4,4'-DIPB in the isopropylation of biphenyl (BP) are examined by changing catalyst amount. The isopropylation of BP gave high selectivity for 4,4'-DIPB at such a low temperature as 200 degrees C even by using large amounts of the catalyst; however, the decrease in the selectivity for 4,4'-DIPB, which accompanies the increase in the selectivity for 3,4'- and 3,3'-DIPB. occurred at such a high temperature as 300 degrees C, and started at lower temperatures with increasing catalyst amount. However, 4,4'-DIPB was highly selective in encapsulated products under all the conditions. These results indicate that shape-selective catalysis occurs inside MOR channels irrespective of reaction conditions, such as catalyst amount and/or reaction temperature, and that the decrease in the selectivity for 4,4'-DIPB is due to the isomerization of 4,4'-DIPB to thermodynamically stable 3,4'- and 3,3'-DIPB at external acid sites. (C) 2009 Elsevier B.V. All rights reserved.