Isobutene is prepared by dissociation of MTBE in the gas phase, by a) fractional distillation of an MTBE-containing stream comprising feed MTBE (I) and a recycle stream (VIII) to give an MTBE-containing overhead stream (II) and a bottom stream (III) having a boiling point higher than MTBE, b) catalytic dissociation of the overhead stream (II) obtained in step a) to give a dissociation product (IV), c) separation by distillation of the dissociation product (IV) obtained in step b) into an overhead stream (V) comprising more than 90% by mass of isobutene and a bottom stream (VI) comprising diisobutene, MTBE and more than 50% of the methanol present in the dissociation product (IV), d) fractional distillation of the bottom stream obtained in step c) under conditions under which the methanol is obtained as bottom product (VII) and more than 99% of the MTBE is obtained in the overhead product (VIII), and e) recirculation of the overhead product (VIII) to step a).