The objective of this work is to study the effects Of SiO2/Al2O3 of HZSM-5, MgO modification and hydrothermal treatment on methylation of 4-methylbiphenyl (4-MBP) with methanol under fixed-bed down-flow conditions. The results show that, with an increase in the SiO2/Al2O3 of HZSM-5 (from 50 to 150, molar ratio), the selectivity to 4,4'-dimethylbiphenyl (4,4'-DMBP) increases from 25 to 79%. MgO modification also increases the selectivity to 4,4'-DMBP, but leads as well to low catalyst activity and rapid deactivation. Hydrothermal treatment not only increases the selectivity, but also improves the stability. When CBV1502 (SiO2/Al2O3 = 150) catalyst was hydrothermally treated at 500degreesC, the selectivity to 4,4'-DMBP increased to 85%. The selectivity to 4,4'-DMBP was further improved to about 90% by the increasing in the amount of mesitylene in the feed as solvent. (C) 2003 Elsevier B.V. All rights reserved.