The reaction of dimethyl- and trimethylbenzenes with dibenzothiophene (DBT), phenanthrene, and a number of other polynuclear aromatics characteristic of light-cycle oils (LCOs) has been explored over zeolites ZSM-5, Y, and Beta. In catalyzing the reaction between trimethylbenzene and DBT, for example, zeolites Y and Beta yielded methyl-substituted DBTs in greater than 80% selectivity at DBT conversion levels exceeding 90%. With smaller ring structures, e.g., in the reaction between m-xylene and benzothiophene, ZSM-5 was similarly effective (but somewhat more prone to aging). It was also shape-selective in its product distribution. Transmethylation provides a simple and effective synthesis route to the polymethyl polynuclear aromatics and thiophenes (PNAs and PNTs, respectively) needed in the reaction between m-xylene and benzothiophene, ZSM-5 was similarly effective (but somewhat more prone to aging). It was also shape-selective in its product distribution. Transmethylation provides a simple and effective synthesis route to the polymethyl polynuclear aromatics and thiophenes (PNAs and PNTs, respectively) needed in the search for improved catalytic routes to very low sulfur distillate and other clean fuels products.