The gas phase synthesis of MTBE (methyl tert-butyl ether) was studied using three series of triflic acid (TFA)-modified zeolites, the parent materials being HY, H-mordenite, and HZSM-5. Impregnation with TFA was found to enhance MTBE synthesis activity only for the large-pore zeolite Y and only up to a certain extent of modification. A high level of TFA modification caused a reduction in activity, apparently due to blockage of the active sites by TFA molecules and extra-lattice Al formed during the modification process. The mechanism of activity enhancement by TFA modification appears to be related to the formation of extra-lattice Al rather than the direct presence of TFA.