Long chain alkyl substituted naphthalenes are produced by alkylating naphthalene with an olefin or other alkylating agent with at least 6, and usually 12 to 20 carbon atoms, in the presence of an alkylation catalyst comprising a zeolite having rare earth cations, and both ammonium and protonic species, associated with the exchangeable sites of the zeolite. The zeolite is usually a large pore size zeolite such as USY. The presence of rare earths and both ammonium and protonic species increases selectivity for production of long chain mono-alkyl substituted naphthalenes in preference to more highly substituted products.