Process for the combined preparation of at least butene and octene from ethene with the following steps: a) providing an inert solvent whose boiling point or boiling range is below the boiling points of the butenes; b) providing a first feed mixture comprising at least the inert solvent and ethene dissolved therein; c) transferring the first feed mixture to a first synthesis; d) in the first synthesis oligomerizing at least a portion of the ethene present in the first feed mixture in the presence of a first heterogeneous catalyst and in the presence of the inert solvent to obtain a first reaction mixture comprising at least the inert solvent, butene, hexene and octene; e) work-up of the first reaction mixture and/or a substance stream based thereon into a low boiler fraction comprising the inert solvent, at least one C 4 -fraction comprising butenes, a C 6 -fraction comprising hexenes, a C 8 -fraction comprising octenes and into a C 8+ -fraction comprising hydrocarbons having more than eight carbon atoms; f) using at least a portion of the low boiler fraction for providing the first feed mixture; g) providing a second feed mixture comprising at least hexene and ethene dissolved in the hexene using at least a portion of the C 6 -fraction; h) transferring the second feed mixture to a second synthesis, wherein the second synthesis is spatially separated from the first synthesis; i) in the second synthesis reacting at least a portion of the ethene present in the second feed mixture with at least a portion of the hexene present in the second feed mixture in the presence of a second heterogeneous catalyst to obtain a second reaction mixture comprising at least octene; k) joint work-up of the second reaction mixture with the first reaction mixture or with the substance stream based thereon. a) providing an inert solvent whose boiling point or boiling range is below the boiling points of the butenes; b) providing a first feed mixture comprising at least the inert solvent and ethene dissolved therein; c) transferring the first feed mixture to a first synthesis; d) in the first synthesis oligomerizing at least a portion of the ethene present in the first feed mixture in the presence of a first heterogeneous catalyst and in the presence of the inert solvent to obtain a first reaction mixture comprising at least the inert solvent, butene, hexene and octene; e) work-up of the first reaction mixture and/or a substance stream based thereon into a low boiler fraction comprising the inert solvent, at least one C 4 -fraction comprising butenes, a C 6 -fraction comprising hexenes, a C 8 -fraction comprising octenes and into a C 8+ -fraction comprising hydrocarbons having more than eight carbon atoms; f) using at least a portion of the low boiler fraction for providing the first feed mixture; g) providing a second feed mixture comprising at least hexene and ethene dissolved in the hexene using at least a portion of the C 6 -fraction; h) transferring the second feed mixture to a second synthesis, wherein the second synthesis is spatially separated from the first synthesis; i) in the second synthesis reacting at least a portion of the ethene present in the second feed mixture with at least a portion of the hexene present in the second feed mixture in the presence of a second heterogeneous catalyst to obtain a second reaction mixture comprising at least octene; k) joint work-up of the second reaction mixture with the first reaction mixture or with the substance stream based thereon.