Abstract: Tertiary-butanol in an aqueous solution containing from 40 to 90 percent by weight of tertiary butanol in homogeneous and liquid phase is dehydrated at a fixed catalytic bed consisting of a strongly acidic ion exchange resin at a temperature from 80.degree. to 150.degree. C. and at a pressure from 5 to 25 bars for the purpose of producing isobutene by dehydrating tertiary butanol.Thereupon the homogeneous, liquid reaction mixture is fractionated in a distillation sector separate from the reaction chamber, the isobutene being separated from the water and from the unreacted tertiary butanol. The unreacted tertiary butanol is returned as an aqueous solution in a circulating flow rate to the reactor under the conditions that the mixture of fresh-fed tertiary butanol and the circulating tertiary-butanol/water mix contains from 40 to 90 percent by weight of tertiary butanol at the reactor inlet.