Shape-selective methylation of 2-methylnaphthalene (2-MN) was carried out over hydrothermally treated HZSM-5 (SiO2/Al2O3 = 83) zeolite catalysts under a fixed-bed down-flow reactor, methanol and 1,3,5-trimethyl benzene (TMB) were used as methylating agent and solvent, respectively. The results show that hydrothermal treatment improves the selectivity to 2,6-dimethylnaphthalene (2,6-DMN) and the stability. HZSM-5 catalyst hydrothermally treated at 550 degrees C followed by acid leaching exhibits 7.1% of 2,2,6-DMN yield after 5 h time on stream (TOS), and 13.4% of 2-MN conversion with the 2,6-/2,7-DMN ratio of 1.8 after 8 h of TOS. (c) 2008 Elsevier Ltd. All rights reserved.