Alkaline earth metal modification of H-mordenite (MOR) was examined in the isopropylation of biphenyl (BP). Of interest was the chemical and textural changes on catalytic properties as a result of the modification. At low loadings of 0.2− 0.5 mmol/g of the metals, the isomerization of 4,4′-diisopropybiphenyl (4,4′-DIPB) was effectively prevented at temperatures as high as 300 °C, resulting in high selectivities for 4,4′-DIPB. Metals reduce the acid amounts by low loadings, and deactivate external acid sites. At the loading of 2 mmol/g, only BaMOR was active with high selectivities for 4,4′-DIPB; however, other metals lost catalytic activity with the increase in the loading. These differences are ascribed to changes of acidity and pore size. The modification of barium with high loading formed moderate acidities, and kept pores open for the access of bulky molecules, resulting in the selective formation of 4,4′-DIPB; however, other metals declined the activities by choking pores for magnesium and calcium or by losing acidity for strontium.