The effects of SiO2/Al2O3 ratio of H-mordenite on the selectivity of encapsulated products in the pores and of bulk products were investigated in the isopropylation of biphenyl. The selectivity of 4,4＇-diisopropylbiphenyl (4,4＇-DIPB) of bulk products was varied with SiO2/Al2O3 ratio. Highly dealuminated H-mordenite gave a selectivity higher than 80%, whereas the selectivity was low for H-mordenite with SiO2/Al2O3 ratio of 10-20. On the other hand, the selectivity of 4,4＇-DIPB in encapsulated DIPB isomers was higher than 85% for all H-mordenites regardless of SiO2/Al2O3 ratio. These results show that all H-mordenites catalyze the isopropylation of biphenyl with high shape-selectivity inside the pore. The low selectivity of 4,4＇-DIPB in bulk products for H-mordenites with low SiO2/Al2O3 ratio was not due to a lack of shape-selectivity of the H-mordenites, but to non-regioselective isopropylation at the external acid sites. Non-regioselective catalysis over H-mordenite with low SiO2/Al2O3 ratio could be induced by choking pores by coke deposition.