The photochemical behavior of the three positional isomers of trans-aminostilbene is reported and compared to that for unsubstituted trans-stilbene. The absorption spectrum of the para isomer displays a single intense long-wavelength band; however the meta and ortho isomers display two less intense bands as a consequence of configuration interaction. All three isomers are fluorescent and display similar solvent-induced shifts of their fluorescence maxima. The fluorescence rate constant of the para isomer is larger than that of the ortho or meta isomer. The para isomer has a short singlet lifetime and high photoisomerization quantum yield, similar to those for trans-stilbene. In contrast, the ortho and meta isomers have long singlet lifetimes and low photoisomerization quantum yields. Isomerization of the para isomer is a singlet-state process, whereas isomerization of both the ortho and meta isomers is a triplet-state process. The lower bound for the barrier for singlet-state torsion of the ortho and meta isomers is 7 kcal/mol. The barrier height is found to be determined by the effects of the amino substituent upon the relative energies of the fluorescent singlet and twisted singlet states.