1-Isopropenylnaphthalene has been dimerized by cationic initiation. The structure of the dimer does not depend on the initiator. The cyclization of the dimeric ion takes place on carbon 8 (peri). The structure has been established both by x-ray diffractometry and by NMR. The complete crystallographic structure has been determined. The dimers of various substituted isopropenylnaphthalenes have been obtained in the same way and have the same structure.