This invention is a process for producing diisobutylene from isobutylene. The process comprises first forming dry sulfonic acid resin by contacting water wet sulfonic acid resin catalyst with a first reaction feed comprising isobutylene under conditions effective to produce tertiary butyl alcohol from the reaction of isobutylene and water, and then contacting the dry sulfonic acid resin with a second reaction feed comprising isobutylene under conditions effective to oligomerize isobutylene to produce diisobutylene.