<p>This invention is a process for producing diisobutylene from isobutylene. The process comprises first passing water through a downflow reactor containing a bed of sulfonic acid resin to produce an effluent stream having a pH of at least 5, then dimerizing isobutylene by contacting the sulfonic acid resin with a reaction feed comprising isobutylene and tertiary butyl alcohol. The downflow reactor comprises a bottom, an inlet located above the resin bed, an outlet located below the resin bed, and inert material in the space from the bottom of the reactor to at least above the outlet.</p>
