NH3-TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo-modified HZSM-5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of H-1 MAS NMR resonance originating from Bronsted acid sites in the zeolites and a distinct splitting of Mo5+ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Bronsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.