The photolysis of 1, 3, 5-cycloheptatriene in the vapor phase led to the formation of toluene and bicyclo [3.2.0] heptadiene- 2, 6. The quantum yield for toluene increased with decreasing pressure, the extrapolated value at zero pressure being unity within experimental error. Inert gases such as helium, xenon, methane, carbon dioxide, nitric oxide, oxygen and diethyl ether affected the quantum yield in qualitatively the same way as pure cycloheptatriene but quantitatively their efficiencies seemed to fall in the order of their boiling points and molecular weights. In particular, xenon seemed to be much more efficient than helium as a quencher. The quantum yield for the formation of bicycloheptadiene was also pressure dependent, although the effect was small and the yield increased with an increase in pressure. Under the most favorable conditions only 5% or less of the excited cycloheptatriene molecules isomerized this way. It is suggested that isomerization to toluene occurs not from an electronically excited molecule of cycloheptatriene but from a vibrationally excited ground state molecule that is formed by the internal conversion of the electronic energy. In contrast, the formation of bicycloheptadiene is believed to occur from electronically excited cycloheptatriene. © 1962, American Chemical Society. All rights reserved.