Quantum yields at 590°K of the cadmium-photosensitized cis-trans isomerization of butene-2 have been determined, with butene pressures ranging from 1.2-13,5 torr, in the presence of 0 to 35.9 torr of benzene. Without benzene, quantum yields close to 0.5 were found for both isomers, and there was no significant difference in the quenching cross sections. With benzene, the yield was larger than one-half the fraction of Cd(53P1) atoms quenched by butene. This result is interpreted by a mechanism in which the 3.8-eV Cd(3P1) atom excites the 3.6-eV ( 3B1u) level of benzene, the triplet benzene molecule then causing the excess isomerization. The rate of reaction of the triplet species with benzene is 0.03±0.04 times that with butene-2, and the ratio of its unimolecular decay rate vs rate of reaction with butene-2 is (5.0±0.7) torr. From the latter figure a lifetime of 0.3×10-6 sec and an upper limit of 6 kcal/mole for the activation energy of unimolecular decay are estimated.