The influence of the acid strength of zeolites upon gas-phase formation of MTBE from methanol and isobutene was studied detached from other parameters such as structure, framework composition, and site density. A series of partially ion-exchanged HY zeolites which had been ion-exchanged to the same extent by Li, Na, and Rb was used, yielding materials with similar concentrations of protons but different acid strengths. All zeolites exhibited similar initial activities for the formation of MTBE at temperatures below 100-degrees-C, although for other acid-catalyzed reactions such as 1-butene isomerization an increase in the activities with increasing acid strength of the zeolites had been previously observed. Steady-state activities, however, were found to be strongly influenced by the different deactivation behavior of the investigated samples. For the partially Rb-exchanged sample, blockage of pores (probably by isobutene oligomers) caused an expressed decrease in rate of reaction during the first 30 min on stream. The presence of excess methanol in the pores is regarded as a possible cause for the similarity of the initial reaction rates.