In this investigation, several Mo/ZSM-5 catalysts with various Mo loadings and various reaction times were prepared, and methane conversion to aromatics without the presence of oxygen was studied by microreactor evaluation, FTIR and XPS. It was found, for the first time, that oxygen treatment before the reaction could shorten the induction period of this reaction. In situ FTIR study indicated the formation of O2-2 species (888 cm-1) and O-Al (670 cm-1) on the surface of the catalyst as a result of the oxygen treatment. The two IR bands shifted to 854 and 659 cm-1 respectively when 18O2 was used. These oxygen species may take part in the initial reaction and shorten the induction time. The XPS study revealed the coexistence of Mo2C and MoO3 species on working catalyst surfaces, and a proper Mo2C/MoO3 ratio (∼ 0.38) was identified for the best aromatization Mo/ZSM-5 catalyst. The active sites for methane conversion to aromatics should include Mo oxide as well as Mo carbide.