Isobutene is prepared by dissociating tert-butanol into isobutene and water over an acid ion-exchange resin arranged as a fixed bed in at least one reactor at a temperature of from 80 to 150.degree. C. and at a pressure of from 5 to 25 bar to obtain a reaction mixture, separating the reaction mixture into isobutene, a by-product, water and at least one mixture of undissociated tert-butanol and water. The reactor is operated pseudo-isothermally, with a temperature difference between inflowing and outflowing streams of less than 15 K.