Photolysis of cis-stilbene in the vapor phase at 170° leads to the formation of trans-stilbene, phenanthrene, and hydrogen. The formation of phenanthrene is not eliminated by the addition of oxygen (35 mm) or nitric oxide (6 mm), nor is the rate affected by large excesses of ether or cyclohexane. It is believed that phenanthrene and hydrogen are formed from cii-stilbene by an intramolecular process which probably proceeds in a single step - the elimination of hydrogen occurring at the same time as the closure of the third ring. The reaction is also observed to occur in the pyrolysis of cis-stilbene at 550°. Qualitative results on the photolysis and pyrolysis of 1,4-diphenyl-1,3-butadiene in the vapor phase are reported. Quantum yields for the formation of phenanthrene in the photolysis of cis-stilbene in cyclohexane solution have been obtained. An earlier report that phenanthrene is a product in solution only in the presence of oxygen has been confirmed. The dissimilarities between the mechanisms of the formation of phenanthrene in the vapor phase and in solution are discussed.