Abstract: Process for the preparation of pure methyl tert.-butyl ether. Isobutene or isobutene-containing hydrocarbon mixtures are reacted with methanol in a molar ratio of 1:1 to 1:2 in the liquid phase at temperatures of between 30.degree. and 100.degree. C. on sulfonated, strongly acidic, macroporous organic ion exchange resins. After separation of the unreacted hydrocarbons, the reaction mixture is distilled under pressure; the methanol-containing distillate formed during the distillation is recycled into the zone of the reaction between methanol and isobutene; and pure methyl tert.-butyl ether is withdrawn from the sump of the distillation column.