A mechanism for the photochemical conversion of 2-vinyl-1,3-terphenyl to 8,9a-dihydrophenanthrene ( Lewis, F. D.; Zuo, X.; Gevorgyan, V.; Rubin, M. J. Am. Chem. Soc. 2002, 124, 13664- 13665) is presented in this study, based on ab initio restricted active space self-consistent field calculations and a molecular mechanics-valence bond dynamics simulation of a model system: the syn isomer of 2-vinylbiphenyl. An extended crossing seam between the ground and first excited electronic states was found to be largely responsible for the efficient photocyclization of the photochemically active syn isomer. This mechanism is nonadiabatic in nature, with an excited-state reaction pathway approaching the crossing region during the initial stage of cyclization. Dynamics simulation shows that this seam is easily accessible by vibrational motion in the branching space, once a small barrier is passed on the S-1 excited-state potential energy surface. Ultrafast radiationless decay to the ground state then follows, and the cyclization is completed on this surface. A second possible mechanism was identified, which involves complete adiabatic cyclization on the S1 surface, with decay to the ground state ( at a different conical intersection) only taking place once the product is formed. Thus, there is a competition between these two mechanisms-nonadiabatic and adiabatics-governed by the dynamics of the system. A large quantum yield is predicted for the photocyclization of the syn isomer of 2-vinylbiphenyl and 2-vinyl-1,3-terphenyl, in agreement with experimental observations.