The objective of this work is to study methylation of 4-methylbiphenyl (4-MBP) with methanol over modified ZSM-5 catalysts under fixed-bed down-flow conditions, especially the effects of zeolite structure, SiO2/Al2O3 ratios of ZSM-5, metal oxide modification, phosphorus modification and hydrothermal treatment. The results show that, over 12-membered ring zeolites, such as HY, HM and H-Beta, there is no sign of selectivity to the target product, 4,4'-dimethylbiphenyl (4,4'-DMBP). Over 10-membered ring zeolite ZSM-5, the catalyst shows moderate selectivity to 4,4'-DMBP. A high SUM ratio leads to high selectivity but low catalyst activity; Both MgO modification and phosphorus modification increase the selectivity to 4,4'-DMBP, but less than 80%. Hydrothermal treatment not only increases the selectivity to 4,4'-DMBP, but also improves the stability. Treatment temperature has great effect on the activity and the selectivity in the methylation of 4-MBP with methanol. The optimal temperature range is between 400 and 550degreesC. The high selectivity over modified HZSM-5 largely results from the depression of 4,4'-DMBP second reaction such as isomerization, dealkylation, as demonstrated by the reaction of 4,4'-DMBP as reactant over modified HZSM-5 and the parent zeolite. (C) 2004 Elsevier B.V. All rights reserved.