Publisher Summary This chapter discusses the nature of shape-selective catalysis in the ethylation and the isopropylation of biphenyl over H-mordenites. 4-isopropylbiphenyl (4-IPBP) is consumed much faster than 3-IPBP in their competitive isopropylation. Selectivity for 4,4′-diisopropylbiphenyl (4,4′-DIPB) in bulk products decreased with the increase of 3-IPBP, however, the selectivity in encapsulated products kept constant. 4-ethylbiphenyl (4-EBP) disappeared much faster than 3-EBP in their competitive ethylation. Selectivity for 4,4′-diethylboiphenyl (4,4'-DEBP) is less than 2%, whereas total selectivity for DEBPs with 4-ethyl group was higher than 65% in both products. DEBPs are predominantly produced from 4-EBP.