The conversion of benzene to toluene and other substituted aromatic products has been investigated at elevated pressure and temperature. Over sodium-free H-Y and H-beta zeolites at 400-degrees-C, 700 psi, and a methane/benzene feed ratio of 8/1, a wide range of methylated aromatic hydrocarbons was produced, with toluene (ca. 50%) being the principal component. Over H-beta zeolite, CH4 and CO2, when co-fed with the benzene reactant, enhanced the yield of aromatic products in comparison to co-fed N2 or H-2. Isotopic tracer experiments, utilizing (CH4)-C-13 co-feed, demonstrated that methane reactant does not become incorporated into the "methylated" aromatic products over zeolite beta and that all of the observed hydrocarbon products were derived solely from the benzene reactant.