A process is described in which butene oligomers are prepared from Fischer-Tropsch olefins, in which the butenes present in a C.sub.4 fraction of Fischer-Tropsch hydrocarbons are oligomerized to obtain dibutene. The dibutene can be fractionated in a fine distillation stage into di-n-butene and di-isobutene. In one embodiment of the process, the ethylene present in the Fischer-Tropsch olefins is dimerized and the dimerization mixture is recycled to the C.sub.4 fraction. In another embodiment, the isobutene present in the C.sub.4 fraction is reacted with an alcohol to give an alkyl tert-butyl ether, and only the remaining n-butene is oligomerized so that di-n-butene is formed as the sole dibutene.