But-2-enes are obtained from C.sub.4 -hydrocarbon mixtures which contain but-1-ene and may or may not contain but-2-enes by catalytic isomerization of the but-1-ene to but-2-enes in an isomerization zone at elevated temperatures by a process in which the isomerization is carried out in combination with a distillative separation zone, the said hydrocarbon mixture being fed to the isomerization zone and/or distillative separation zone, the isomerization mixture obtained from the isomerization zone is passed into the distillative separation zone, a but-1-ene-containing fraction is removed above the lower third of the distillative separation zone and passed into the isomerization zone, and the but-2-enes or a fraction containing these are or is removed in the lower third of the distillative separation zone.