Abstract: This invention concerns an improved and novel catalyst for preparing methyl tertiary butyl ether (MTBE). This invention is advantageous in that the reaction of methanol and isobutene takes place such that the catalysts exhibit levels of isobutene conversion as high as 98%, and the MTBE selectivity reaches as high as 98%. The improved catalysts comprises of a crystalline aluminosilicate zeolites, particularly MFI-type zeolites which has been treated with aluminum fluoride in the ratio 1 gram to 10 grams of zeolite with 0.5 gram to 5 grams of aluminum fluoride. A specific application of this improved and novel catalyst is reacting methanol and isobutene in a molar amount of about 0.1 mole to 10 moles of methanol per mole of isobutene, in the presence of said catalyst in a batch reactor, at about 70° C. to about 100° C., and a pressure of about 1 bar to 33 bar, to obtain MTBE product.