of 1.3.5.7-Cyclooctatetraene etc. (Part 11) absorption spectrum [A. Langseth and S. Brodersen, Acta Chem. Scand. 3, 778 (194911. The latter would, however, not explain the decrease of absorption in the range 260 -280 nm observed in the present experiments at T z 650 K.Product distributions in the UV photolysis of cyclooctatetraene vapour have been measured. Two parallel reaction channels leading to decomposition into benzene + acetylene and to isomerization to dihydropentalene or semibullvalene have been observed. Equal isomer yields have been found at pressures of 2 -3 Torr; semibullvalene formation is, however, favoured at increasing pressures. and dihydropentalene formation becomes more important at decreasing pressures. Absolute quantum yields and specific rate constants k ( E ) for isomerization and decomposition of cyclooctatetraene have been determined under steady state photolysis conditions. Direct measurements of k ( E ) under collision-free conditions have been performed in the laser flash photolysis of cyclooctatetraene. In addition the steady state photolysis of dihydropentalene (main primary reaction product semibullvalene) has been studied; again quantum yields and specific rate constants have been determined.Die Produktverteilung bei der UV-Photolyse von Cyclooctatetraen in der Gasphase wurde gemessen. Dabei wurden die parallel ablaufenden Reaktionen zu den Zerfallsprodukten Benzol + Acetylen und den Isomeren Dihydropentalen bzw. Semibullvalen beobachtet. Beide Isomere entstehen mit etwa gleicher Ausbeute bei Drucken um 2-3Torr; bei ansteigendem Druck wird die Bildung von Semibullvalen und bei geringerem Druck die Bildung von Dihydropentalen bevorzugt. Absolute Quantenausbeuten und spezifische Geschwindigkeitskonstanten k ( E ) fur Isomerisierung und Zersetzung von Cyclooctatetraen wurden unter den Bedingungen der stationaren Photolyse bestimmt. Mit Hilfe der Laser-Blitzlicht-Photolyse von Cyclooctatetraen wurden zudem direkte Messungen von k ( E ) unter stoofreien Bedingungen durchgefuhrt. Weiterhin wurde die stationare Photolyse von Dihydropentalen untersucht. Als uberwiegendes Primarprodukt wurde das Isomer Semibullvalen gefunden. Auch in diesem System wurden Quantenausbeuten und spezifische Geschwindigkeitskonstanten bestimmt.Photolysis and photoisomerization reactions following electronic excitation of molecules are governed in many cases by excited electronic state kinetics. Often, however, also fast internal conversions to the electronic ground states are possible such that unimolecular reactions of vibrationally highly excited ground state species occur. The latter systems can be compared with thermal activation experiments and serve as ideal test cases for unimolecular rate theory. Polyenes like cycloheptatriene [l -31, substituted cycloheptatrienes [3, 41, cyclohexadiene and hexatrienes [5] have been found to be such examples. In the present work it was our intention to investigate whether cyclooctatetraene (COT) reacts in a similar way, i.e. whether the reactions after thermal an...