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 which contains cations having a radius of at least 2.5 .ANG.. Cations of this size may be provided by hydrated cations such as hydrated ammonium, sodium or potassium cations or by organoammonium cations such as tetraalkylammonium cations. The zeolite is usually a large pore size ze USY. The presence of the bulky cations in the zeolite increases the selectivity of the catalyst for the production of long chain mono-alkyl substituted naphthalenes in preference to more highly substituted products.