Process for preparing oligomers by continuous oligomerization of butenes, wherein a) a feed stream 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 used (hereinafter referred to as feed stream 1), b) this feed stream 1 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, c) the oligomers obtained in b) are separated off and optionally passed to a further work-up and the remaining residual stream is fed to work-up by distillation, d) isobutane is separated off by distillation from the residual stream, e) the isobutane-depleted stream obtained after the work-up by distillation d) (hereinafter referred to as feed stream 2) is reacted to form oligomers.