Abstract: The invention relates to process for dimerizing olefinic hydrocarbon feedstock, to hydrocarbon compositions and to fuel components produced by the process. According to the invention, fresh olefinic hydrocarbon feedstock is fed to a reaction zone of a system including at least one reaction zone and at least one distillation zone. The olefinic hydrocarbon feedstock is contacted with an acidic catalyst in the presence of an oxygenate at conditions in which at least a part of the olefins dimerizes. The effluent from the reaction zone is conducted to the distillation zone where dimerized reaction product is separated from effluent, and at least one flow comprising oxygenate is withdrawn from the side of at least one distillation column. The flow is circulated from distillation zone back to dimerization. The reaction mixture is recovered and optionally hydrogenated to form a parafinic reaction product.