Methyl tertiary butyl ether is prepared from tertiary butyl alcohol and methanol in a plural stage process by (a) reacting tertiary butyl alcohol with methanol in a primary MTBE reaction zone containing a cationic ion-exchange resin catalyst to form a primary reaction product, (b) fractionating the primary reaction product in a primary distillation zone to provide a first lighter, lower boiling distillation fraction comprising isobutylene, methanol and methyl tertiary butyl ether and a first heavier, higher boiling distillation fraction comprising methanol, tertiary butyl alcohol and water, c) contacting the first heavier distillation fraction in a second stage reactor containing a second stage acidic, fluoride-treated Y-zeolite having a silica:alumina ratio of 100:1 to 10:1 and a unit cell size of from 24.20 to 24.45 .ANG. or a fluoride-treated silicoaluminophosphate (SAPO) molecular sieve having a pore size of from 5 .ANG. to 8 .ANG., under conversion conditions including a temperature of from 20.degree. to 300.degree. C. and a pressure of from 0 to 1000 psig to form a second stage reaction product rich in isobutylene and also containing methyl tertiary butyl ether; and (d) converting the isobutylene to additional methyl tertiary butyl ether.