Process for preparing isobutene from tertiary butanol

In a method for the production of isobutene (I) by cleavage of tert.-butanol (II) at 80-150[deg]C and 5-25 bar on a strongly acid ion exchange resin, followed by separation to give (I), by-products, water and mixture(s) of unreacted (II) and water, the reactor is operated quasi-isothermally with a temperature difference of less than 15[deg]K between the feed and the product streams. Independent claims are also included for (1) isobutene-containing mixtures obtained by this method; and (2) an isothermal reactor system for use in the method, comprising at least two separate reaction zones containing a solid-bed catalyst as above, provided with devices for maintaining isothermal conditions, consisting of a separation zone between two reaction zones by means of which at least part of at least one of the products of the equilibrium reaction is vaporized by changing the pressure or temperature, separated from the reaction mixture and taken out of the reactor, while the rest of the mixture is transferred to the next reaction zone, and also optionally other devices by means of which the temperature or the pressure in the next reaction zone can be adjusted exactly to the value in the preceding zone.