Irradiation of all-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) in acetonitrile (AN) gives ctt- and tct-DPH by relatively inefficient pathways but mainly via the singlet excited state. Assuming that twisted singlet excited state intermediates partition equally to cis and trans ground state double bonds leads to the conclusion that the major nonradiative decay process of the singlet excited state of ttt-DPH ((1)ttt-DPH*) is direct decay to the ttt-DPH ground state, phi(nr) = 0.54. If CH stretching vibrations serve as accepting modes in this decay process, deuterium substitution should profoundly attenuate it. We report a comparative study of perhydro-ttt-DPH with di- and tetradeuterated ttt-DPH (ttt-DPH-d(n), n = 0, 2, and 4) involving deuteration of one and both terminal triene double bonds. NMR and HPLC analyses give k(H)/k(D) = 1.36 +/- 0.1 for terminal bond isomerization at 20.0 degreesC. The very small changes in tau(f) and of are consistent with such a kinetic deuterium isotope effect on the rate constant for terminal bond isomerization. The temperature dependencies of tau(f), phi(ctt), and phi(tct) for (1)ttt-DPH-d(0)* give energy barriers for torsional relaxations that are well above the 2A(g)-1B(u) energy gap. The major radiationless decay process is not related to trans --> cis photoisomerization, is barrierless within experimental uncertainty, is completely insensitive to deuterium substitution, and occurs in the ns time scale. The mechanistic implications of these results are discussed.