The acidic properties of both HZSM-5 and SiO2 Supported MoO3 and carburized MoO3 have been investigated by FTIR spectroscopy. Deposition of Mo caused the consumption of Bronsted acidic OH groups of HZSM-5 as shown by the changes in the v(OH) region of the spectra and also by pyridine and low temperature CO adsorption measurements. Carburization of the sample did not result in regeneration of acidic OH groups of the zeolite. Mo reacted with OH groups during its deposition on SiO2. The results of both pyridine and CO adsorption measurements did not indicate the generation of Bronsted acidic sites on MoO3/SiO2 and carburized MoO3/SiO2. Lewis acid sites are formed, however, upon the deposition of Mo. Carburization led to stronger Lewis centers, which are probably the active sites-together with the carbide phase-in methane aromatization on MoO3/SiO2. (C) 2003 Elsevier B.V. All rights reserved.