The vapor-phase photochemistry of both 1,3-cyclohexadiene and 1,3,5-hexatriene can be treated as a single system since the photolysis of one gives rise to the other and the remaining products are common to both systems. In the present study, the pressure dependence of the products has been used to demonstrate that a classification of the reactions in this system as those that are common to both photochemistry and thermochemistry and those that occur only in the photochemistry parallels the behavior with pressure. It is argued that the first class of reactions which include 1,3,- cyclohexadiene+hv→benzene+H2 →2C2H 2+C2H4, →polymerization, 1,3,5-hexatriene+hv→[1,3-cyclohexadiene]*→benzene+H2, occurs from a vibrationally excited ground-state molecule which originates from the initially formed excited singlet state by internal conversion. These reactions have been found to decrease with an increase in the pressure of the parent olefin or with the addition of inert gases such as xenon, carbon dioxide, propylene, or ether. Such data when coupled with the calculated lifetimes of the excited singlet states show that collisional quenching of the excited singlet-state molecules is unimportant at pressure below ~8 mm. The trends observed with an increase in temperature support this general picture. Copyright © 1963 by the American Institute of Physics.