Catalytic functions of Mo/HZSM-5 toward methane aromatization are examined under various activation and reaction conditions. Molybdenum ions are necessary in the activation of methane. High benzene yields of around 7% are obtained with a selectivity of more than 96% at 973 K over a 2 wt % Mo/HZSM-5 catalyst in a tubular reactor. Pretreatment with hydrogen is found to be beneficial to the initialization of aromatization. The aromatization activity increases upon decreasing the Si/Al ratio in HZSM-5, indicating an acidity dependence. This is supported by the fact that a Mo catalyst supported on cesium-exchanged ZSM-5 losses its catalytic aromatization activity at all. The effective coordination between molybdenum ion sites and Bronsted acid sites in the framework of ZSM-5 is important in the activation of the stable C-H bonds in methane and the eventual aromatization to form benzene. The constant Ols binding energy in the XPS spectra of both fresh and used catalysts suggests that coke formation does not occur on the SiO4 tetrahedra but rather on the molybdenum sites supported on the external surface of ZSM-5.