The objective of this work is to study the effects of hydrothermal treatment conditions on the methylation of 4-methylbiphenyl (4-MBP) with methanol under fixed-bed down-flow conditions. The results show that hydrothermal treatment temperature has a marked effect on the activity and selectivity in the methylation of 4-MBP with methanol. 4-MBP conversion decreases and the selectivity to 4,4'-dimethylbiphenyl (4,4'-DMBP) increases with an increase in the hydrothermal treatment temperature. The optimal temperature range is between 400 and 550 degreesC. After hydrothermal treatment, the catalytic selectivity to 4,4'-DMBP is up to 70%, while the 4-MBP conversion is about 12%. The high selectivity over hydrothermally treated ZSM-5 largely results from the suppression of 4,4'-DMBP secondary reactions such as isomerization and dealkylation, as demonstrated by the reaction of 4,4'-DMBP as reactant over hydrothermally treated ZSM-5 and the parent zeolite.