Long chain alkyl substituted naphthalenes are produced by the alkylation of naphthalene with an olefin or other alkylating agent possessing at least 6 carbon atoms, usually 12 to 20 carbon atoms, in the presence of an alkylation catalyst comprising a zeolite having rare earths associated with less than 50 % of the exchangeable sites of the zeolite. The zeolite is usually a large pore size zeolite such as zeolite USY. The partial exchange with rare earths increases the selectivity of the catalyst for the production of long chain mono-alkyl substituted naphthalenes in preference to more highly substituted products.