The invention relates to a process for dimerizing lower, olefinic hydrocarbons. In particular, the present invention concerns a process for dimerizing C.sub.3 , C.sub.4 and C.sub.5-olefins with a medium pore zeolite under process conditions allowing selective dimerization According to the invention, fresh olefinic hydrocarbon feedstock is fed to a reaction system including at least one reaction zone and at least one separation zone. The olefinic hydrocarbon feedstock is contacted with an acid catalyst at conditions in which at least a part of the olefins dimerizes. The effluent from the reaction zone is conducted to the separation zone where dimerized reaction product is separated from said effluent.