The synthesis, structure, and properties of octalene are described for the first time. This annulenoannulene, accessible in many steps from a tetrahydronaphthalene, is a lemon yellow hydrocarbon that is very air-sensitive. Independent structural proof was obtained by crystal structure analysis of a triazolinedione adduct. Examination of the 13C-NMR spectra at +120, +25, and −150°C ruled out the form with a central double bond and the aromatic form. The temperature dependence of the spectrum indicates mutually independent inversions of the cyclooctatetraene and the cyclooctatriene fragments. The peripheral π-bonds undergo shifts above about 80°C. The transition state is planar.