The present invention relates to a process for preparing oligomers consisting mainly of repeating units derived from 1- or 2-butene from a hydrocarbon stream consisting substantially of branched and linear hydrocarbon compounds having 4 carbon atoms, and comprising olefinic branched and linear hydrocarbon compounds having 4 carbon atoms (C4 starting stream) by a. in step a), separating the C4 starting stream into a fraction consisting mainly of linear hydrocarbon compounds having 4 carbon atoms (l-C4 fraction) and a fraction consisting mainly of branched hydrocarbon compounds having 4 carbon atoms (b-C4 fraction), by contacting the C4 starting stream with a membrane which is easier to pass for linear hydrocarbon compounds having 4 carbon atoms than for branched hydrocarbon compounds having 4 carbon atoms, b. in step b), optionally after removing butanes, oligomerizing the olefinic hydrocarbon compounds having 4 carbon atoms present in the l-C4 fraction, c. in step c), subjecting the olefinic hydrocarbon compounds having 4 carbon atoms present in the b-C4 fraction to one of the following steps: c1. reaction with methanol to give methyl tert-butyl ether (step c1) c2. hydroformylation to give substantially isovaleraldehyde (step c2) c3. polymerization to polyisobutylene (step c3) c4. dimerization to 2,4,4-trimethyl-1-pentene (step c4) c5. alkylation, substantially to form saturated hydrocarbon compounds having 8 or 9 carbon atoms (step c5).