A process for the dimerization of isobutene wherein tertiary butyl alcohol is used to enhance the selectivity of the catalyst to the dimer is disclosed wherein the tertiary butyl alcohol is removed from the diisobutene product by water wash. The water/TBA stream is then subjected to reextraction to remove the TBA for recycle to the dimerization reactor. The dimerization is preferably carried out in a reactor wherein the pressure is controlled to maintain the reaction mixture at is boiling point. Additional dimerization is carried out in a distillation column reactor that acts as a debutanizer to separate the unreacted isobutene form the product and the tertiary butyl alcohol.