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The isomerization of t-stilbene (stilbene hl2) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d 12) is produced by deuteration of the two ethylinic hydrog~ ns o. nly (stilbene dz). Complete deuteration of the phenyl rings (stilbene d 10) has rather httle mfluence on the decay of the jet-cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h 12 and d2 Using similar parameters the calculations consistently place the stilbene d2 and stilbene dlO curves in the wrong order. Our results suggest extensive but not complete …