The dehydrogenation of methane was carried out over a Mo/ZSM-5 catalyst. It was revealed that the purity of the methane was very critical for the evaluation of the catalyst activity. In order to study the phenomenon, the effects of the addition of O-2, CO2, CO or H-2 to the feed were investigated. A small amount of O-2 increased the amounts of aromatic compounds and CO produced. The addition of H-2 scarcely affected the conversion of methane, but it prevented the deactivation of the catalyst, i.e., benzene production remained constant during a 6 h test.