Process for producing methyl tert-butyl ether (MTBE) by reacting methanol with the isobutene present in a C.sub.4 cut in a proportion from 10 to 60%, wherein said cut, admixed with methanol is contacted, in a first reaction zone, with an acid catalyst in the conditions of etherification of isobutene by methanol unit 93 to 98% of the isobutene has been converted, the reaction product is fractionated under at least 10 bars to a bottom fraction consisting essentially of MTBE and a top fraction which is contacted, in a second reaction zone, with an acid catalyst, under the conditions of etherification of isobutene by methanol, until at least 99% of the initial isobutene has been converted, the resulting product is washed with water and separated into a residual C.sub.4 cut which is discharged and an aqueous phase wherefrom methanol is recovered by distillation.