The addition of a few percent of water to methane feed significantly improved catalytic performance in the methane dehydroaromatization reaction on 6 wt% Mo/HZSM-5 catalysts at 0.3 MPa and 998-1073 K. Exactly three times as much hydrogen as CO was produced from H2O and methane, suggesting that the reforming reaction was accompanied with the dehyrocondensation reaction of methane. However, addition of water above 2.6% resulted in a sudden drop of catalytic activity after several hours of time on stream. Coke, a main inhibitor of the reaction, produced a lesser amount with increasing water concentration in methane feed. The HZSM-5 structure of Mo/HZSM-5 examined by XRD and Al-27-MAS-NMR techniques remains intact even after the reaction at 1073 K in the presence of water except for a high water concentration of 2.6%, where framework aluminum migrates to extraframework positions, which may be responsible for the sudden drop of the catalytic activity. Small Mo2C clusters, an active species of the reaction, were always observed on used catalysts by EXAFS analysis. Stabilities of Mo2C and C in a water atmosphere were estimated from equilibrium calculations and discussed in conjunction with the reactivity of coke formed on the catalyst during the methane dehydroaromatization reaction. Based on the above results, the sudden drop of catalytic activity at an excess addition of water may be explained by the two consecutive reactions of CH4 = C + 2H(2) and C + H2O = CO + H-2 (reforming reaction as a whole). These two consecutive reactions proceed whenever coke is present on the catalyst but not on the catalyst having no coke. Therefore, water can pass through on the catalyst having no coke and eventually almost all aluminum atoms in the lattice position move out by the action of water. Then, the catalyst can not promote the reforming reaction and also the dehydrocondensation reaction of methane. (C) 2003 Elsevier Inc. All rights reserved.