Process for producing a 1-butene-containing C4-hydrocarbon stream (1-C4null stream) from a 1-butene- and 2-butene-containing C4-hydrocarbon stream (1- and 2-C4null feed stream) whose 1-butene content is lower than that of the 1-C4null stream, by a) feeding the 1- and 2-C4null feed stream and a 1-butene- and 2-butene-containing C4-hydrocarbon stream (1- and 2-C4null recycle stream) whose 1-butene content is lower than that of the 1-C4null stream and which has been produced by means of step (b) below into a distillation column and taking off the 1-C4null stream and a 2-butene-containing C4-hydrocarbon stream (2-C4null stream) whose 1-butene content is lower than that of the 1- and 2-C4null feed stream and of the 1- and 2-C4null recycle stream from the distillation column (step a) and b) producing the 1 - and 2-C4null recycle stream from the 2-C4null stream by bringing the 2-C4null stream into contact with an isomerization catalyst which catalyzes the conversion of 2-butenes into 1-butene in a reaction zone (step b).