A process for the dimerization and cyclization of isobutene to form, as preferred product, paraxylene in which the whole of the product of isobutene dimerization (including unreacted feedstock) is contacted with a catalyst for the cyclization of the isobutene dimer. The process is operable in a single dimerization/cyclization stage or in separate dimerization and cyclization stages with no intermediate separation of products. Optionally, the isobutene feedstock is provided by dehydrogenation of isobutane, unreacted isobutane in the resulting feedstock being carried through the dimerization/cyclization and eventually recycled.