The structures of Mo/HZSM-5 catalysts with various molybdenum loadings were studied by means of XRD, IR, UV diffuse reflectance spectroscopy, TPR, and ammonia adsorption and desorption measurements. Both the BET surface areas and the acidities of catalysts decrease with an increase in molybdenum loading in the catalyst. The threshold of a monolayer dispersion of molybdenum is about 5 g of molybdenum per 100 g of HZSM-5 zeolite. Methane conversion under nonoxidizing conditions over Mo/HZSM-5 catalyst was tested. It was found that the catalyst with a molybdenum loading of 2-3 wt% exhibits optimum activity for the dehydro-oligomerization of methane to aromatics. Modifications of the 2% Mo/HZSM-5 catalyst with lithium or phosphorus cause a decrease in the acidity of the catalyst as well as in the catalyst activity. Addition of lithium shifts the selectivity toward ethylene at the expense of the yield of benzene. It is also demonstrated that the molybdenum oxide species are partially reduced by methane during the reaction. The removable lattice oxygen of molybdenum oxide oxidized adsorbed CHx species to CO, which results in a side reaction to the catalytic oligomerization of methane to aromatics. The diminution of acidity of the catalyst and the blockage of the channels of HZSM-5 zeolite due to deposited carbon may be the main reasons for the deactivation of the catalyst. The methane oligomerization reaction is proposed to be catalyzed by molybdenum species located in the zeolite channels together with the Bronsted acid sites of HZSM-5 zeolite. A synergistic effect between these two kinds of centers plays an important role in the catalysis of the title reaction. Ethylene is identified to be a primary product while benzene is a final product in the dehydro-oligomerization reaction of methane.