This paper describes characterization studies of Mo-HZSM-5 zeolite catalysts active for the non-oxidative conversion of methane to benzene. FTIR, Al-27 and Si-29 NMR evidence is presented for migration of molybdenum into the zeolite pores during catalyst calcination at high temperatures. Mo K-edge EXAFS confirms that calcination produces highly dispersed oxomolybdenum or molybdate species which are converted to a molybdenum carbide phase under reaction conditions. Factors determining catalyst performance are discussed.