Slurry phase alkylation of biphenyl with propene was carried out using H-mordenite (HM) catalysts with SiO2/Al2O3 ratios of 10, 15, and 20. The HM(10) catalyst exhibited low selectivity for 4,4’-diisopropylbiphenyl (4,4’-DIPB) production compared with the HM(20) catalyst. When the external acid sites were poisoned by treatment with tributylphosphite, however, HM(10) became the most selective catalyst for 4,4’-DIPB production. The high selectivity of HM(10) was related to its low activity for isomerization of 4,4’-DIPB. Propene was selectively consumed by alkylation on HM(10), while undesirable side reactions of propene occurred on HM(20). These results are understood by taking acid strength into consideration.