Acidity investigation of fresh and used Mo/MCM-22 catalyst for methane aromatization is described. For determination of Bronsted acidity, connected with bridged OH groups, and the Lewis acidity, related to Al, quantitative analysis was carried out using FTIR technique and employing d(3)-acetonitrile as the probe molecule. Acidity changes for both the parent zeolite and the Mo-containing zeolite were analyzed in individual periods of the catalyst development in the methane stream, i.e. during the activation period (up to 70 min), at the optimum performance (between 70 and 150 min), during catalyst deactivation (up to 10 h) and after its regeneration in oxygen. Both bridged hydroxyls (concentration 0.21 mmol/g) and Lewis acidic sites (two types; total concentration 0.14 mmol/g) were present in the parent H/MCM-22 zeolite. Molybdenum incorporation induced a decrease in the concentration of bridged OH groups to about one half of the original value following calcination at 450 degreesC, while the content of Lewis sites decreased by 20%. The Mo/MCM-22 catalyst at the maximum catalytic performance for benzene production was shown to contain less than 0.01 mmol/g of bridged OH groups. For the parent H/MCM-22 zeolite, treated under the same condition, the Bronsted acidity drops to 50% of its original value and further decreases with time on stream. The Lewis acidity of the parent and Mo-containing zeolites initially decreased by about 15-20% in methane atmosphere, but was nearly constant during the experimental run between 1.5 and 10 h. Regeneration in oxygen produced only a very limited restoration of the OH groups in both the parent and Mo-containing samples, while the formation of new internal SiOH groups indicated deeper destruction of the zeolite lattice. The catalytic activity was partly restored, but not to its original value.These findings support earlier concepts of the role of OH as a trap for molybdenum species during MoOx spreading in the opening steps of the catalyst preparation, while they are contradictory to the assumed role of high concentration of the Bronsted acid sites in zeolite for the aromatization reaction itself. (C) 2003 Elsevier B.V. All rights reserved.