A method for dimerizing isobutene, wherein, in dimerizing conditions, the isobutene is brought into contact with a porous cation exchange resin comprising a styrene polymer, which is cross-linked with divinyl benzene, and any sulphonic acid groups adhering to the polymer. The harmful deactivation of this catalyst, which is caused by the accumulation of the oligomers and polymers of isobutene and other C.sub.4-olefins in the cation exchange resin catalyst, is essentially decreased by selecting the cation exchange resin from a group including cation exchange resins, the acid,capacity of which is 4.7 equivalents/kg at a minimum, and the portion of divinyl benzene units of which is 5% by weight at a minimum and smaller than 20% by weight.