The invention relates to a method for obtaining isobutene from conjugated hydrocarbons. Said method comprises the following steps; a) placing C4-hydrocarbon with a C3- or C4-primary alkanol; b) reacting isobutene in conjugated C4-hydrocarbons with C3- or C4-primary alkanol in the presence of a heterogeneous catalyst to obtain the corresponding isobutene-based tertiary ether; c) causing fractionation of the accumulated reaction mixture into non ethereal C4-hydrocarbons with a low boiling point and isobutene-based tertiary ether with a high boiling point by using a fractionating tower, said C4-hydrocarbons being removed by the head while the isobutene-based tertiary ether accumulated on the bottom is fed into a reactor; d) causing fission of said ether, thereby obtaining isobutene and the corresponding primary C3-C4-alkanol; e) distilling the blending resulting from step d) by using another fractionating tower, and removing the isobutene as an overhead. Step a) is carried out in an area (4C) containing the reaction ingredients in which the catalyst is placed to complete step b); the area (4C) is part of a fractionating tower (4); in said area (4C9 is effected a reaction fractionation; C3- or C4-alkanol is fed upstream from area (4C) of the fractionating tower (4), and the conjugated C4-hydrocarbons downstream from said area (4C).