H-mordenites are very highly shape-selective catalysts in the alkylation of biphenyl with tert-butanol under liquid phase conditions using n-decane as a solvent with a selectivity to 4-(tert-butyl)biphenyl above 98% and to 4,4'-di(tert-butyl)biphenyl near 100% to give 24% yield of 4,4'-di(tert-butyl)biphenyl using H-MOR(45).