Deuterated ethenylanthracenes, 1-, 2-, and 9-(ethenyl-(Z)-2-d)anthracene, and 2-(ethenyl-(E)-2-d)anthracene ((Z)-1-, 2-, and 9-EAD, and (E)-2-EAD, respectively) undergo photochemical Z-E isomerization under thermal activation. The temperature dependence of their isomerization quantum yields and the temperature independence of lifetimes of their T-n <-- T-1 absorptions indicate that EADs isomerize adiabatically between the Z and E isomers in the lowest excited triplet state by overcoming an activation barrier (E(a) = 19-46 kJ mol(-1)) of the perpendicularly twisted geometry. The activation parameters of isomerization, Delta H double dagger (17-42 kJ mol(-1)) and Delta S double dagger (-40 to -113 J K-1 mol(-1)), decrease in the order of 2-, 1-, and 9-isomers; however, Delta G double dagger (50-54 kJ mol(-1)) is almost constant irrespective of the substitution position of the anthracene nucleus.