The acid sites of Mo/HZSM-5 and the interaction between Mo species and HZSM-5 have been characterized by in situ FT-IR spectroscopy using pyridine as the probe molecule. The results reveal that Mo species are mainly located on OH groups associated with silanol groups and bridged hydroxyl groups, Si(OH)Al. This is shown by the decrease in the intensities of their stretching modes at 3740 and 3618 cm(-1). On a 6 wt% Mo/HZSM-5 sample, the relative intensity of the band at 3618 cm(-1) decreases about 80%, and the band at 3740 cm(-1) decreases about 65%. Thus, the Mo species preferably locate on the Bronsted acid sites after calcination at 773 K. FT-IR characterization of adsorbed pyridine shows that both Bronsted and Lewis acid sites are present in all Mo/HZSM-5 samples. The relative integrated intensities of the bands of pyridine adsorbed on the Bronsted and Lewis acid sites, as well as the ratio of the Bronsted and Lewis acid sites concentration, [L]/[B], are estimated as a function of Mo loading. The strength distribution, investigated with stepwise evacuation under different desorption temperatures, is broader on Lewis acid sites than on Bronsted acid sites. An increase in the intensity of the band at 1549 cm(-1) is observed with the 15 wt% Mo/HZSM-5 sample, suggesting that new Lewis and Bronsted acid sites are generated. Combining the FT-IR study with previous catalytic evaluation, it is concluded that Mo/HZSM-5 is a bifunctional catalyst and that a certain amount (about 60%) of the original Bronsted acid sites on HZSM-5 kept free on the surface may be necessary for its good catalytic performance in methane dehydrogenation and aromatization.