Abstract: Polycyclic aromatic compounds, such as biphenyl, are alkylated with an alkylating agent, such as propylene, in the presence of an acidic mordenite zeolite catalyst under conditions sufficient to produce a mixture of substituted polycyclic aromatic compounds enriched in the para alkylated isomers, such as p,p'-di(isopropyl)biphenyl. The novel acidic mordenite catalyst is characterized by its silica/alumina molar ratio, its porosity, and a Symmetry Index. The p,p'-disubstituted isomers of polycyclic aromatic compounds are useful as monomers in the preparation of thermotropic, liquid crystal polymers.