Abstract: In the alkylation of naphthalene using an alkene reactant to produce monoalkyl naphthalene, the inhibition of polyalkyl and specially dialkyl naphthalenes by the use of a mixed protonic acid catalyst consisting of methane sulfonic acid and an active P.sub.2 O.sub.5 containing acid, utilized in about a 2:1 to 1:2 ratio with an optimum ratio of about 1:1. The reaction is carried out preferably under anhydrous conditions with respect to the mixed catalyst, and the products show present utility as emulsion breakers in petroleum chemistry as well as other surface active agents.