The liquid phase synthesis of methyl tert-butyl ether (MTBE) from methanol and isobutene over II-Beta and US-Y zeolite catalysts was studied in the temperature range 30-120 degrees C. Up to 100 degrees C, commercial II-Beta zeolite samples with small crystal size were more active than acid Amberlyst-15 (reference catalyst) and noticeably more active than US-Y, confirming results obtained under vapour phase conditions. The influence of methanol/isobutene (MeOH/IB) molar ratio, pressure, and space time on the conversion and MTBE selectivity was investigated. At optimized reaction conditions, MTBE yields of 85-90% can be reached with zeolite Ii-Beta as well as Amberlyst-15. On zeolites, side reactions of isobutene are more important than on Amberlyst-15, necessitating operation at MeOH/IB ratios higher than 1:1. For the same reason, at high conversion on Ii-Beta, the MTBE yields are more sensitive to contact time compared to Amberlyst-15. On II-Beta zeolite, no deactivation was observed during a period of more than 50 h on stream at 65 degrees C, 1.4 MPa pressure, and a WHSV of 14 h(-1). The catalytic activity of the zeolites is related to the external specific surface area, and to the concentration of bridging hydroxyls and silanol groups in the mesopores. A zeolite H-Beta sample with a Si/Al ratio of 36 has an optimum silanol and bridging hydroxyl content leading to stoichiometric methanol and isobutene adsorption, highest activity and MTBE yields.