Abstract: A light gas oil, consisting predominantly of alkylated naphthalenes plus minor amounts of thiophene, quinoline, indan, and related naphthalene type carbonaceous compounds, derived from the liquid by-product from olefin production by pyrolysis in steam of a hydrocarbon fraction, is converted to mononuclear aromatic hydrocarbons by a two stage process in which said naphthalene type carbonaceous compounds are first hydrogenated at tetralinizing conditions saturating only one of the two aromatic rings of the feedstock. Said first stage hydrogenation also hydrodesulfurizes and hydrodecontaminates said feedstock. The tetralanized desulfurized normally liquid effluent from the first stage hydrogenation is hydrocracked in the second stage, said hydrocracking being in the presence of a catalyst featuring a stabilizing metal component on an acid-modified mordenite having a silica to alumina unit mol ratio within the range from 15 to 70 as a result of acid leaching of mordenite.