The mechanism of the photoisomerization of cis,cis-l,5-cyclooctadiene to tricyclo[3.3.0.02,6]octane has been studied in the vapor phase as a mercury-sensitized reaction, and in ether solution with cuprous chloride as catalyst. In the gas phase the use of oxygen as a radical scavenger failed to eliminate the reaction. The reaction in the condensed phase was followed by the use of 1,5-cyclooctadiene-d12. No loss of the deuterium label was detectable in either the tricyclooctane that was formed or in the cyclooctadiene that was recovered at conversions ranging from 23 to 70 %. The reaction is best formulated as an intramolecular process and not as a free-radical reaction as proposed by Baldwin and Greeley. © 1966, American Chemical Society. All rights reserved.