An isotopic study utilizing methane-C-13 shows that the product toluene and xylenes formed from its reaction at high pressure (5.5 MPa, 400 degrees C) with benzene, over Cu/beta and H-beta zeolites, contain substantially methyl-C-13 substituents. It is therefore concluded that direct methylation of benzene with methane is possible under these conditions in a batch reactor. However, a second mechanism can also be recognized in which methylbenzene is formed by fragmentation and rearrangement of benzene without addition of methane gas.