A process for preparing oligomers by continuous oligomerization of butenes is described, wherein a) a feed stream 1) comprising 1-butene and 2-butene in a total concentration of from 10 to 70% by weight and from 10 to 60% by weight of isobutane is reacted until more than 60% by weight of the 1-butene comprised in the feed stream 1 but less than 50% by weight of the 2-butene comprised in feed stream 1 have been converted into oligomers. b) The oligomers obtained in a) are separated off and optionally passed to a further work-up and the remaining residual stream is fed to work-up by distillation. c) Isobutane is separated off by distillation from the residual stream, and d) the isobutane-depleted stream obtained after the work-up by distillation c) is reacted to form oligomers.